Posted on 18 Comments

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

Happy AC

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 grayfurnaceman for make 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 as anytime at: or comment below. We would love to help you out and have your business!

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

  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

Leave a Reply

Your email address will not be published. Required fields are marked *