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Problem: My AC heat pump unit makes a humming noise even when it is not running?

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Problem: I ran our heat pump, air conditioner all night and it did great, but when I went outside this morning the unit was making this buzzing/humming sound.  It still works correctly, but it makes that humming noise even when the unit is not running.  In other words, when the thermostat has the unit in the off mode it makes the noise, but when the temperature in the house rises and the thermostat turns the unit on everything is fine.

Answer: Many heat pumps have a reversing valve solenoid that is energized (24 volts AC) all the time in the cooling mode. This reversing valve solenoid is energized whether the unit is running or not. If the solenoid is getting old, or if the solenoid is out of alignment, or loose then the buzzing, humming sound will occur. Try to switch your thermostat to the heating mode and see if the hum is still present. If the hum is still present then you might have a noisy outdoor transformer. If the noise is so loud that it is bothersome then you may need to replace the reversing valve solenoid or transformer.

Another thing I forgot to mention. You might want to test your transformer and make sure you are getting 24 to 28 volts AC out of the secondary on the transformer. If the transformer is producing low voltage (under 24 volts) then this would cause a loud humming, vibrating sound. Please click here if you would like to see the low voltage transformers we sell. Below we have a Youtube video made by Supercool Slike Rules which shows how to test a reversing valve coil. Thanks to Supercool Slide Rule for making this informative video! We also have another video made by grayfurnaceman which shows how a reversing valve works. Thanks to grayfurnaceman for making this informative video! Please make sure all the power is turned off before testing or inspecting your air conditioning parts. If you have any questions please email us anytime at: or comment below. We would love to help you out and have your business!

You Can Search Our Site for More Troubleshooting Problems with Answers, Heating, Air Conditioning Parts and Supplies in the Google Search Box Below:

39 thoughts on “Problem: My AC heat pump unit makes a humming noise even when it is not running?

  1. I just got a NEW Bosch Heat Pump (BOVB-36HDN1-M18M) and Ceiling Mount uncased Air Handler (GAS24G000HTS00) installed and it works great except one thing. The unit clicks on at the thermostat, the Air Handler starts blowing slightly cool air but when the heat starts flowing, a muffled humming sound can be heard near the air handler and in the adjacent wall to the air handler. It lasts about 2-5 minutes, then the sound goes away as the slightly cooler air ends the cycle. This happens only when the Heat Pump is in heat mode only. When cooling there is no humming at all. Please help, the technicians have been out here 8Xs and they have not been able to figure it out.

    1. Hi Maryann! I do not know for sure what the problem is, but it sounds like you possibly have a noisy TXV valve or refrigerant metering valve. Your service contractor tech should be able to find out where the noise is coming from by listening with a mechanic’s stethoscope or a long screwdriver held up to his/her ear to listen. I would think that the refrigerant flow through your TXV expansion valve that is used in the heating mode is noisy and probably needs to be replaced. The valve might be partially stopped up, have a restriction, and could be causing this noise. Here is a YouTube link that shows and explains some noises that are caused by mini-split systems: If your installed did not evacuate the new system properly then there could be air or impurities in the system that is causing this noise. Bosch and your installing company should warrant their work since this is a new installation. I hope that you can get this problem fix free of charge. Steve

  2. I recently installed a multi head system (MITSUBISHI 42KBTU MULTI-ZONE SYSTEM MOD#
    MXZ-5C42NA2, one 15 kBTU cassette in the living room MOD# SLZ-KA15NAR1.TH, four 9 kBTU cassettes in the bedrooms). The cassettes cycle on an off to maintain their designated set points but in between (when the fan and cooling stops) there is a constant humming noise that basically never goes away until the cassette / thermostat is set to off. This is mildly annoying during the day but disruptive to sleep over night. I’ve heard the cassettes need to run a defrost cycle from time to time but I don’t expect it to run nonstop for hours, especially when the runtime is actually pretty low (to keep my bedroom at 78 over night in temperate weather it doesn’t need to run much). I love my new system aside from this incessant noise but the installer simply said that the units make noises it’s normal. Is this normal?

    1. Hi Alana! I am very sorry that you wasted your time writing to me. I do not know anything about Mitsubishi multi-zone systems. I most certainly would think that the unit/units should not make any humming noises. I have heard low voltage transformers and reversing valve solenoids make noises before. The solution to the noise was to replace or tightened up the transformer or reversing valve and the noise stopped. I would like to suggest that you call another contractor to find and stop the noise or call Mitsubishi and ask them about the noise. I am very sorry that I can not be of any help. Maybe someone will see your post and can help you out. Steve

  3. Hi Steve

    I have a 5 yr old confortmaker heat pump. Only in the winter when the defrost kick on not in the summer with ac on . I hear a humming noise that penetrates thru the cinder block brick wall from heat pump outside . This humming noise travels up above to my livingroom from the refrigeration lines that are insulated in a 1ft pvc pipe running thru the cinder block wall from heat pump At the base of the condo basement to the blower/coil . The refrigeration lines are insulated under the condo where you can also hear the same noise humming up close in the refrigeration line only when defrost kicks on

    The capacitor is new. The motor Quiet , The reverse solenoid valve and contactors are all good no noise issues.What can be done at the heat pump regarding where the refrigeration line come out where you hear the noise up close to eliminate this humming noise. Can the heat pump lines internally in the heat pump be reconfigured avoiding the refrigeration line humming noise when defrost kicks on thank you.

    1. Hi Kent! I am sorry that I do not have any suggestions other than try and find out exactly where the noise is coming from and then separate the lines from the block, brick wall with rubber isolators or foam insulation to stop the noise and vibration. I had to do this with my AC because of the refrigeration lines were coming in direct contact with the floor joints and out outside brick wall. I had to crawl under the house while the AC was running make some adjustments in the refrigeration lines and stuff some foam black rubber insulation in between the floor joints and lines to stop the noise and vibration. Sorry that I can not give you an easy solution to this problem. Steve

  4. I have a goodman heat pump. theres a humming noise at the compressor and the contactor does not pull in, so unit doesn’t come on …. if you push in contactor manually fan and compressor come on… now if I disconnect the blue wire to right side of contactor the humming goes away. ive replaced the contactor and the cap and still the same..could it be the mother board.

    1. Hi Frank! Sounds like you have a low voltage problem since your contactor is not getting 24 volts to the contactor coil. We cover this problem in the following posts. We also have some good Youtube videos. You will need a voltmeter to test and see where the problem is located. Here are the posts: and If you have any questions please let me know. I hope you can easily find and fix the problem. Steve


    1. Hi Betty! If the blower door on your air handler is getting real hot where it is almost too hot to touch then I would recommend that you check with a voltmeter or an Amprobe to see if the electric heaters are cutting off when they are supposed to. If you do not have a meter then maybe you can tell by how fast your service electric meter is spinning to determine if the heaters are off or not. The new digital electrical usage meters are hard to determine if more or less power is being used. You might have a stuck sequencer that is keeping the heaters on when the blower is off. The noise that you might have heard could be the heat pump going into defrost mode and the noise may have been refrigerant running through your refrigerant lines. I am just guessing of course. If you have a unit that has a TXV valve for refrigerant metering then the valve could be noisy. The refrigerant flowing through the lines can be noisy sometimes. If you do not have the tools to test the unit then I would recommend having the unit checked out by a licensed, good HVAC contractor. Sorry that I can not be of any help. Steve

  6. I have a 3 year old Daikin heat pump. It sounds like it’s tapping out morse code when the fan in the unit is not running. I pulled the outdoor breaker and the noise is till there. When the unit is running the noise is gone and after it stays off for a while the noise, likewise, eventually disappears. The unit cools just fine and has no other issues I know of. I had a Technician here but he could never duplicate the problem. He checked all the likely electrical causes and everything checked out fine. Googling the problem brought me to your page about reversing valves and I was wondering if these symptoms sounded like an issue at the reversing valve or would you consider something else? I have run out of ideas and just need some direction here. Thanks very much

    1. Hi Jim!
      The only thing that I can think of that it might be would be a check valve in the refrigeration lines that makes a tapping noise when the unit is off and the refrigerant is moving and equalizing in the system. I would not worry about this unless the sound becomes too annoying. This does not sound like a reversing valve problem. If the reversing valve is working and switching between heating and cooling then I would not recommend that you worry about it. I hope you have a great and blessed weekend! Steve

      1. Thank you for the quick reply, Steve. That all makes real good sense. This does put my mind at ease. I appreciate your help.

  7. Hi, thanks for the help. I bought an Amana PTAC second hand; cleaned it up and installed. Heat and Cool work well, but once I hit power button to turn it off; it continues to hum and instantly starts to frost over on the copper water lines.

    1. Hi John! Sorry, I do not know much about PTAC units (Packaged Terminal Air Conditioners). Since it is like an air conditioning unit it sounds like the compressor is not shutting off and continuing to run all the time without the fan running since the lines are frosting over. I would like to suggest that your look at the contactor (if the unit has one) or the on/off control to see if they are sticking in the closed, ON position. Sorry that I do not know much about PTAC’s. I hope you can get it fixed soon! Steve

  8. I had a Fujitsu mini split installed about 4 years ago I am now getting humming noises coming from the indoor unit for the last two months and that’s with the unit being off.I even turned power off from the disconnect the funny thing is these humming sounds only happen on sunny days and never at night and on cloudy days .i even tried Fujitsu tech support they had no answer.

    1. Hi Marc! I am sorry, but I do not know anything much about mini-split units. I would guess that it might be a noisy thermostatic expansion valve “TXV” where the metered refrigerant is making the noise since you say that the noise only happens on sunny days where the heat from the sun would create higher pressure in the system and cause more refrigerant to flow even when the system is off. The only way to fix this that I know would be to replace the thermostatic expansion valve. This is only a guess. The only other thing that I would look for would be a noisy low voltage transformer. I am sorry that I can not give you any help on this. I hope you can easily find and fix the problem. If you do find the answer please let us know so it can help others. Thanks! Hope you have a great day! Steve

  9. I’ve got a American Standard heat pump. My house is staying warm this winter but something very weird is going on with outside unit. The fans will come on if I turn up thermostat but when fans stop it makes a humming or buzzing noise All the time. I turn my thermostat to off and it still humming. The only way it stops if I kick the breaker off. Ok now the tech assumed it was defrost board so we replaced it. Still humming. Checked Freon and levels are great. Replaced fan board and sensor…still humming. Then he thinks maybe the fan motor is going out so he replaced the fan motor and still humming. My daughter turned off the thermostat in house cause it got up to 82 degrees today when I had thermostat set on 68. When I came home the house was extremely hot and very warm air was coming out of the vents. I thought well she didn’t turn off like she said. Sure enough the thermostat was off, heat coming thru vents and unit is just humming away. I’m about to pull my hair out . What else could I replace???

    1. Hi Beth! I really do not understand which part of your heat pump is humming the outdoor unit or indoor air handler. If it is the outdoor unit then I would check the reversing valve relay because the reversing valve solenoids hum sometimes on outdoor units. If it is the indoor unit I would check the low voltage transformer and blower motor capacitor to make sure they are not humming. A weak motor run capacitor will make a motor hum. Since you say that your home thermostat is not working properly in holding the correct temperature I would suspect shorted thermostat wires or a faulty thermostat. You might need to inspect the thermostat wires to see if they are shorted anywhere or replace the thermostat wires with new wires. The humming should be easy to find by going to the area or where the hum is coming from to see which part is making the noise. I would recommend making sure you are getting a minimum of 24 volts out of your low voltage transformer because low voltage will cause hums and chatter in low voltage parts. You might need a new low voltage transformer. I hope you can easily find and fix the problem. Steve

  10. Hello!
    I just had a new American Standard system installed in my home. The air handler is outside and the furnace is in the attic. When the ac stops I hear a high pitched squealing/humming noise in the bathroom (the wall is next to the unit outside). The sound lasts for approximately 10 min. I find it annoying and when the house is quiet I can actually hear it outside the bathroom. The guys who installed it said it’s normal. I think it needs to go away!! Help please.

    1. Hi Karen! So sorry to hear you are having the squealing humming noise in your unit. I am sorry, but I have no idea where the noise is coming from because I am not there to see and find the noise. Most certainly your installing contractor should find the noise and fix it. This is not normal for a unit to squeal and hum loud. This noise could be coming from a TXV valve in the unit, a noisy fan motor or refrigerant moving through your system. I would tell your contractor that it is so loud that it is bothering you and this is not normal for any unit, most certainly not normal for a new unit. I hope you can get if fixed soon. Please let me and everyone know if you find out where the noise is coming from and when they get it fixed. I hope! Thanks! Steve

  11. Good Day, Steve. I appreciate you taking time and making the effort to respond to all of us ‘plain folks’ inquiries. I, too, have a buzzing/humming 24/7 outside unit heat pump , but only in a/c mode. No noise when in heat mode. Would it be that low voltage transformer, and/or the reversing valve solenoid, that are loose, or misaligned? Or should I assume either or both need replacement?


    1. Hi LR! More than likely the buzzing sound is in the 24 volt reversing valve solenoid. On most units other than Rheem, Ruud the reversing valve solenoid is energized the whole time the unit is in the air conditioning mode. So 24 volts is being supplied to the solenoid all the time the thermostat is on “Cool”. Yes, you might have a misalignment, could be loose or might need replacing the reversing valve solenoid valve. Sorry we do not sell these solenoids. I would try tightening the solenoid if you can. I hope you can easily fix the humming noise. Steve

  12. Hi Steve,

    My split aircon heating when on initially it will run then after about 15 to 20 mins, the fan stop running ,but the outside unit is still running.
    We waited for it to kick in even after 4 to 5 hours still no air but outside unit is still running.

    We try the rest of the feature such as fan and cool aircon all working well.

    We did call a aircon tech he came and change a valve was working for a day then it back to square one.

    wondering what is the likely error with the heat pump.

    Your advice is appreciated.



    1. Hi Billy! If your condenser fan quits running in the middle of a cycle it will cause the compressor to over-heat and go off on internal thermal over-load. Sometimes it takes many hours even over-night for some compressor to cool down where they will restart again. I would suggest that you find out why your condenser fan is stopping. This could be caused by a weak fan run capacitor, a motor that is going bad or a defrost control board relay that controls the fan could be going bad. You might need a new defrost control board. Now if your indoor fan on your furnace or air handler is going off and the outdoor unit is still running then this can be caused by a bad contactor, a bad defrost control board or a defective thermostat. Shorted thermostat wires can also cause this problem. I would recommend fixing this as soon as possible because over-heating the compressor and going off on high head is very hard on the compressor. I hope you can easily find and fix the problem. Steve

      1. Correction i hear a humming sound inside the livingroom from comfortmaker heat pump when running not hunning sound

      2. Hi Kent! This could be a transformer inside your air handler humming or a blower motor that is trying to run with a weak capacitor. I would like to suggest that you check the motor run capacitor to make sure that it is in good condition. Some low voltage transformers become loose and need to be tightened down. Some blower motors hum when the capacitors are weak or the windings in the motor are getting weak. Sometimes replacing the capacitor with a new capacitor or running the motor at a higher speed will eliminate the hum. I hope you can easily find and fix the problem. Steve

  13. my condensing unit heat pump/ a/c unit does not hear or cool . during the winter i used the emergency heat with natural gas to get us through winter but with summer here i need my condensor to work. my symptoms are when i switch the thermostat to heat or cool, my condensor fan spins,. while the condensing fan is spinning i hear a buzz sound for 1-2 seconds every 15 seconds, like something wants to start but can’t . i replaced the capacitor and checked the contactor and they seem ok. mayb the compressor? wiring or mechanical?

    1. Hi Brian! This sounds like your compressor is locked and will not start. You might need to try a hard start booster. If the booster does not work then you will probably need a new compressor or a new unit. We discuss this problem like you are having in the following post: I would also suggest that you turn off the power and check the three wires that connect to the compressor. Sometimes the wires are broken or burnt. I hope you can find an easy and inexpensive solution to your problem. Steve

  14. I have a split unit. Inside/Outside. Runs great, cools great. My question is when it reaches the desired temperature and shuts off I hear a humming sound on the inside. Even if I switch it to the off position I still get the humming sound on the inside. Is that normal? Will that run my electric bill up? We had a guy come and he rewired it and serviced it because wet just bought the house. Afterwards we noticed the humming and he said that was normal and fine. Just trying to see if that’s true. Thanks

    1. Hi Stephanie! More than likely you have a noisy low voltage transformer. Many times this noise can be corrected by tightening the transformer mounting screws. The transformer supplies low voltage power to your furnace and air conditioner. The transformer being on in normal and it is necessary for the transformer to be on all the time. It uses very little electricity so it will not run up your electric bill. It is not normal for it to hum loudly and you might try tightening up the mounting on the transformer or replace it with a quiet transformer. Thanks for asking this question. Steve

  15. Thank you for what you had to say about the heat pump not necessarily needing to be removed if the noise isn’t too bothersome. I know that noises don’t bother me too much, but I think that it might grate on my husband’s nerves. We will have someone come look at it before it can get worse.

    1. Yes, I think it would be a good idea to have someone come make sure the heat pump is charged up right and everything is working correctly. Thanks so much for your kine words. God bless you and your family. Steve

  16. why coming humming sound inside the carrier duckt unit.

    1. Most of the time if you have a split system (not a packaged all-in-one unit) and you have a hum coming from the indoor unit it is usually caused by a noisy furnace low voltage transformer that might need tightening up or replacing. Blower motors that have a weak capacitor will sometimes hum when the blower is running. If you are getting the hum when the blower is running I would suggest you have your blower motor capacitor tested to make sure it is in good condition. I hope you can easily find and fix the problem. Steve

  17. Mine recently started humming even while off ,but not when i turn it to heat. However, when i have it off, it hums on and off. And starts humming when in power save mode and it shuts off automatically, it still hums.

    1. Hi! Most of the time humming comes from the low voltage transformer or the reversing valve relay. Sometimes tightening the transformer mounting screws or the reversing valve solenoid will stop the humming noise. Sometimes the transformer or solenoid have to be replaced. We have a post about this problem on the following page: You might need to call a service tech to find and fix the problem. Most wall units are hard to work on because they are bulky and heavy. I hope you can easily find and fix the problem. Steve

    2. The inside and outside units hums for awhile after it shuts off. Not for long maybe 2-5 minutes then goes off. I also just noticed when heater is running some times it blows cool air and sometimes it blows out hot.

      I understand it could be the transformer from what I read, but is that the same problem for the difference in the air temperature?

      1. Yes, this sounds like it could be a transformer humming, but I do not know why the sound would go away after 2 to 5 minutes because the transformer is on all the time. I would suggest that you open the air handler access door and see if you can hear where the hum is coming from. It could be the liquid refrigerant running through the refrigeration lines and after 2 to 5 minutes the refrigerant has equalized on both high and low sides then the hum stops. Since the unit is blowing intermittent cool and warm air then I would suggest that you get your unit checked out by a tech with gauges to see why it is doing this. This is not good on the compressor if the unit is not charged properly or if you have a restriction in the system causing the problem. I hope you can get it fixed soon! Steve

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