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Problem: Outdoor heat pump unit makes a loud sound in the winter? What is this noise?

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Question: Hi, thank you.  We have a standard all-electric attic blower with outdoor condenser Carrier AC/heat unit.  In the winter, in the middle of the night, the unit outside will make a loud decompression/discharge sound, almost like firing a torpedo.  Not a squeak or rattle or gurgle, but a one-time bwwwuuuusshhhh.  Once or twice a night, when it’s pretty cold out.  loud enough to wake us up. Chris, Buckeye Arizona.

Answer: Thanks so much for your interest in our website and the opportunity to do business with you! From what you describe this sounds like you probably have a heat pump unit and the noise that you are hearing is probably the heat pump going into defrost mode. When a heat pump goes into defrost the reversing valve will shift into cooling mode so the outdoor unit will be defrosted. During this defrost time the heat pump defrost control should turn your indoor electric heaters on so you do not get air conditioned cold air coming out of the registers. If it is cold enough outside you will probably see smoke (moisture) raising above the outdoor unit. I have had heat pump customers call me saying it looks like their outdoor unit is on fire when actually this is normal. Yes, going into defrost mode on some heat pumps can be noisy when the reversing valve shifts the outdoor fan turns off and your indoor electric heaters are energized. I hope this is the problem because this is normal from a heat pump. If it becomes too noisy you might want to call a service tech to come and make sure the operating pressures and refrigerant levels are right. Below we have a real good YouTube video made by Air Comfort Heating and AC which explains how a heat pump system defrosts and what to expect. I hope that you have a blessed day. Steve

25 thoughts on “Problem: Outdoor heat pump unit makes a loud sound in the winter? What is this noise?

  1. Thanks for this informative video! I thought my unit was bad because of the noise it was making but your video sounded the same way. Big relief for me. Thank you!

    1. Thanks so much for your kind words Jo! I am very glad to hear we were able to help you out. Steve

  2. I have a Carrier 30 (I think that means a 3 ton) anyway. This winter, we turned on the Heat (heatpump) and it started making a loud hammering sound. We called the service company… he said at 14 years old, it was time to replace it… the whole thing. What I want to know, is it possible that the valve is stuck half way…. so because I had nothing to loose… I disconnected the power and removed the cover…. I took the coil off the plunger of the solenoid. It seemed stiff so using a pulling wedge, I pulled it out a little. What I need to know, should I have pushed it in. Is it Ok just to turn it on and see what happens? I don’t really care about being able to run the Heat Pump… just the AC. If you don’t know if it’s supposed to be out or in… how can I find out these things?

    1. Hi Paul! Yes, it is possible that the reversing valve could be stuck half way between heating and cooling. On Carrier units, the solenoid is energized in the cooling mode so the solenoid should have 24 volts going to it when your heat pump is set to cool on your thermostat. I am sorry that I do not know what you are referring to when you talk about a plunger. The solenoid uses magnetics to cause the valve to shift. I am sorry that I can not answer your question. I hope that you can get the heat pump fixed and going. Steve

  3. RUUD Unit We bought a replacement defrost control board because our outside unit was freezing up, this was not the exact board but one that said it replaced my model and looks identical. Now our unit has been running for about a week I notice the that it’s colder inside, so I check the thermostat and it’s reading 4 degrees behind the set temp. I go outside and notice I have ice on the lines that feed from the house to the unit, so I look at the unit in the attic and the lines are all frosted and the side of the unit has ice on it, I open the unit and the evaporator coil is frozen, so I’ve been running the fan only to defrost the coil, I checked the defrost board before I started defrosting and the lights were blinking which shows to be normal on the board legend, after I started defrosting and turned on the heat, both led lights are off… I’m confused, any help would be greatly appreciated. I have a feeling it will be freezing up again.

    1. Hi Scott! Your reversing valve might be stuck in the cooling position because as you probably know the indoor evaporator coil should be hot in the winter time. Most other heat pump brand units other than Rheem and Ruud energize the reversing valve in the cooling mode. I believe that Rheem Ruud energizes the reversing valve in the heating mode unless they have changed since I was doing service back in 2005. So you could have a reversing valve solenoid coil that is bad or a thermostat on your wall might not be energizing the reversing valve in the heating mode. Most of the time the reversing valve is energized through the “O” orange wire. This could also be a broken or loose wire or a heat pump that is low on refrigerant charge. You might have a faulty defrost thermostat, but this is unusal for the evaporator to freeze up in the winter time unless the heat pump is stuck in the cooling mode. The defrost thermostat should have been replaced when you installed the new defrost board. Sounds like a versing valve problem. I would suggest that you get a professional to look and troubleshoot unless you are good with troubleshooting heat pumps. God bless you and your family. Steve

  4. Is there any way I can send you a phone video with sound of what our heat pump makes. It’s not the same sound that’s in your movie. Thanks!

    1. Hi Donna! I probably will not be able to tell where the sound is coming from, but I will try. You are welcome to send a phone video to our email address. Our email address is arnoldservice@gmail.com I hope you have a great day! Steve

  5. Hello- I have a 5 ton Carrier heat pump that is 11 years old. Every so often, it will make a loud pop, like a firecracker. It seems to be heating and cooling ok. Any ideas of what might be the cause?

    1. Hi Clay! If you are referring to the outdoor unit making a pop then the only thing that I can think of that might cause this would be ice forming on the outdoor coil expanding and contracting when the heat pump is running. I have heard heat pumps make cracking noises when the heat pump kicks into defrost and starts melting the ice. Sorry that I can not give you any other guesses. I hope you have a great day and week! Steve

  6. Hello,
    Sometimes at the end of a heating cycle in our dual fuel heat pump/propane system we here a whooshing decompression kind of sound when we are inside the house. The outside temp can be just around freezing or so. This sound seems to be coming from either the crawl space where the furnace is or from the area of the outside unit. Seems like a new sound. We have a 5 ton Carrier which is 3 yrs old and the decompression type sound lasts just a few seconds. Sounds like air brakes hissing on a bus or truck. Is this worth calling our HVAC people out?

    1. Hi Chris! This sounds like the reversing valve is shifting back and forth. The only time that the reversing valve should shift on a heat pump in the winter is if the heat pump is starting a defrost or ending a defrost cycle. If your heat pump is starting or ending a defrost cycle then this is normal, but if it happens too often and not during a defrost cycle then I would suggest getting the heat pump checked out. Yes, this could be a thermostat problem, a defrost control, defrost thermostat, low on charge or just real cold outside. I recommend turning heat pumps to straight aux heat (emergency heat) when the temps are below 20 degrees outside for long periods of time (1 or more days). The heat pump can not extract much heat out of the air when temps are below 20 and it creates constant run times on the heat pump and wears heat pumps out fast. When outdoor temps are above 20 then turn the heat pump back on off aux (emergency heat). Steve

  7. Thanks Steve for making this video. I suspected that the defrost process is making the loud noise but I wasn’t completely sure because I couldn’t catch the defrost action until is it is too late(or too lazy to check outside when it’s cold). Also, the strange thing is that I haven’t notice the this noise until recently with our 7 or 8 years old Carrier system.

    1. Glad to hear that our post on heat pump unit noise in the winter was able to help you out. God bless you and your family today and always. Steve

  8. Thank you SO MUCH, Andrew, for the tutorial on weird heat pump sounds! Haven’t got a Carrier, as you demonstrated with, but sounds were basically identical. What a big help and $$ saver–won’t have to call for service this time. And if I ever do call them, at least I won’t sound like a complete novice homeowner!

  9. Hello!
    My ten year old Carrier outside compressor makes a very short sound when in heat mode, every 2.5 minutes. When running or not….
    Seems to work fine.
    What gives?

    1. Hi Chuck! I am sorry, but I do not know what this sound could be since I am not there to hear what it sounds like. I know that refrigerant running through the system will make noise, and sometimes transformers and solenoid valves on heat pumps make noises. Sorry that I can not help you out. Steve

  10. Hi,
    Thank you so much for the video explaining clearly how the heat pump works. We have a Gibson heat pump which cools and heats our upstairs. Especially in winter it makes such a god awful noise. the noise is like a circular saw going through a 2 x 4 wood and wakes me up most of the times. I thought that something is wrong with our unit but your video explains it in great length. Thank you so much!

    1. Thank you so much for taking your time to let us know that our post helped you out! We hope you have a blessed and Happy Holiday Season! Steve

  11. My 5 ton Payne heat pump usually runs flawlessly, has now for three years. Yesterday on a 98 degree day with a cut on temp of 79 with a 5 min delay on compressor to cycle on, it came on and sounded like a very loud ramped up horn inside. Starting lone then a creshendo to a higher tone that lasted about 20 seconds. My impression is that the copper lines were vibrating. It was very loud! It has not repeated and seems back to normal now. Ideas?

    1. Hi Marc! This sounds like a possible restriction in your metering device which could be a TXV (thermostatic expansion valve) or restrictor pin depending on what kind of metering device your system has installed. I would suggest having the pressures and super heat checked out if this continues. You might want to turn the power off to your unit and clean the outdoor coil because if the coil is dirty this will cause high pressure in the system. A slow fan motor, which could be caused by a weak capacitor could cause high pressure too. I would like to suggest having the capacitor checked to make sure it is in good shape. If you want me to look up parts then please send me your unit’s product number. I hope that after cleaning the coil and testing the capacitor that the noisy stops. Steve

  12. I was actually doing online research regarding heat pumps and came across this website and seems to be very informative on the Q’s & A’s. I myself am an tech and have a question. I have a heat pump system and recently my contactor went, I replaced it immediately and the new one lasted for a week. Any reason why this new contactor failed so quickly?

    1. Hi Mr. Ramos! Most of the time contactors fail prematurely because the contactor coil is not receiving a minumum of 24 volts. If the coil voltage is under 24 volts the contactor contacts will chatter, arc and wear out prematurely. Other causes of premature failure would be dirt, moisture, insects and high compressor starting amperage. I would suggest checking the amperage the compressor is running and make sure the contactor is rated much higher that the RLA (Running Load Amps.) The higher amperage rated contactor the better. Best of luck in finding the problem. Steve

      1. I just got a brand new Rheem out and inner unit installed. The the contactor failed within a month. The installer thought this was unusual (he said it arc’d), but just swapped out a new one. My reversing valve is extremely loud, like a horn…. I am going to call another tech and worries me, but any thoughts here. We reused old thermostat wires, can it be related?

      2. Hi Jamie! Since the contactor went on within a month I would suggest checking the voltage going to the controls in your heat pump outdoor unit. If the contactor is not getting at least 24 volts AC to the contactor coil then this would cause contactor chatter as cause the contactor to go out prematurely. This low voltage could also cause the reversing valve to not seat properly and cause a loud noise. You might have a slight short in one of the old thermostat wires which is robbing power to the contactor and other controls. A loose wire connection can cause this problem too. I hope you can easily find and fix the problem. This all should be under warranty. Steve

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