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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

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

  1. I have a Carrier 2-stage heat pump and my unit makes the sound you describe in your original post – a loush Swooshhhh. I liken it to opening a Coke bottle under pressure. Thing is it doesn’t do it in the winter as you describe above. It does it in the summer when the A/C has been running. Every time the A/C is running and shuts off, the sound happens. Sometimes it’s not big, sometimes very loud. We just installed this unit in August ’20 so it’s barely 18 months in service. Last summer I had the install team out to do PMs on it and asked about it. Told it’s normal. But I’d like a second opinion. Also note that in first winter (’20’) the heat pump would make loud noises (different noises). Was told that the heat pump had too much pressure for my air handler (which was installed at the same time) and they recommended not using. They disabled the heat pump features and we just heat strips in the air handler. Have you ever heard of that?

    1. Hi Mr. Hebert! It is normal for the heat pump to make a swishing noise whenever the reversing valve changes. Since I have not been doing service since 2005 and you having a 2 stage heat pump where the operation is more complex and controlled by a computer board then I have no idea when the reversing valve is supposed to be changing. In the old heat pumps before 2005 other than Ruud and Rheem units the reversing valve is energized all the time that the thermostat is in cooling mode. If it is in cooling mode then it is constantly energized and did not move until the thermostat is set to heating mode. In heating mode, the reversing valve is not energized at all except when the heat pump goes into defrost mode to defrost the heat pump coils. There is no reason to place your heat pump in emergency heat mode unless the temp is going to be below 32 degrees F or lower for several days. If the temp is above 32 then I would keep it in heat pump mode. This will raise your electric bill lots if you go to straight electric heat. Thanks so much for asking this question. Let me know if you have more questions. Our email is

  2. Hello,
    I just installed a new 3.5 ton Rudd heat pump. Runs fine except when it goes into defrost. 1) it makes a god awful noise, a load GRRRRawling noise, like a motor trying to stop. I know the piping pressures have to equalize in order for the system to reverse, but I normally hear a swissssssshing noise like you have said. I have never experience a load grrawling noise like this. any ideas? 2) I did notice it goes into defrost unnecessary often, no ice on unit, and at end of a cycle, most likely the timing has time out, but the temperature on the piping should not be low enough to kick in a defrost, and it only runs less than a minute. First time I have tried a Rudd, maybe it is something to do with Rudd units?

    1. Hi Stan! I have heard some of the units make some pretty load sounds when going into defrost. About the only way to tell if it is not normal would be to install some gauges and watch the gauges when the heat pump goes into defrost. You might have a defrost thermostat issue since the heat pump seems to be defrosting when it really does not need to defrost. You might want to test it with some ice and see if the defrost thermostat is closed all the time or is closing when exposed to the ice at 30 degrees. Here is a link to a really good Youtube video that explains how to test defrost controls and how it works: The only difference that I know in Rheem, Ruud over other heat pump brands is the Rheem, Ruud energizes their reversing valve in the heating mode where other brand name units energize the reversing valve in the cooling AC mode. So on the Ruud, Rheem heat pump when the unit goes into defrost the reversing valve would not be energized (no 24 volts to the reversing valve solenoid) so the heat pump would go into cooling mode and melt the ice on the heat pump. Sounds like you might have either a defrost board problem or a defrost thermostat problem. Hopefully, the heat pump is under warranty if you do need parts. I hope this helps you out. Steve

  3. I have a heat pump in a home we just bought. I heard a sound like a water pipe in the wall thumping. It happens late at night or very early morning. It starts out slow then quickens then slows down again and stops. The duration seems to be lengthening over time. I went out and felt one of the the pipes and it felt like it was surging like it had air in the line. The pipe had insulation around it also. The other two had none. These lines come from the unit in the attic and go outside the the fan assembly. It’s the line that goes to the fan assembly that’s making the noise. It’s loud enough to wake me up. Any assistance would be greatly appreciated.

    1. Hi Dennis! I am sorry to hear that you are having this noisy heat pump problem! I am also sorry, that I really have no idea why and where the noise is coming from other than maybe the compressor vibration is traveling through your refrigeration line set and into the walls of your home. My air conditioner’s scroll compressor was vibrating the floor in our home when it was running. This was very annoying and sometimes loud! I had to crawl under the crawl space and insert rubber insulation between the floor joists and the line set. This extra support and rubber insulation helped the noise to stop. I do not know if your line set is accessible, but I hope that it is so you can do what I did to fix our noise. You can find the rubber insulation tape that I used on the following page: I used plumber’s pipe strap and sheet metal screws to support and screw the refrigeration pipes to our ductwork. Here is a link to the plumber’s pipe strap I would also like to recommend that you get your heat pump checked out by a licensed HVAC contractor to make sure that your heat pump is charged up properly and running efficiently. I would tell the HVAC repair tech about your noise. I hope you can find and fix this problem easily and inexpensively. Steve

  4. 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

  5. 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

  6. 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

  7. 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 I hope you have a great day! Steve

      1. Steve, were you able to determine what’s going on with Donna’s unit? I ask because I have a similar situation. I am accustomed to the sound of my unit going into defrost mode. However, in cold temperatures it makes a different noise. It sounds more like an extremely loud wail; it’s loud enough to get the attention of our neighbors. Our home is new and the unit was manufactured in May 2020. I’m trying to determine what the issue is before the warranty expires and prevent it from becoming an expensive problem. I too can send video/audio, if needed. Thanks

      2. Hi Kenny! I have not heard what was going on with Donna’s unit. I would like to suggest that you call the company that installed the heat pump and tell them about the noise that you hear when the heat pump defrosts. I am not familiar with late-model, super high-efficiency heat pumps because I have not been doing service work since 2005. I am 68 years old. I would not think that it is normal to make such a loud wailing noise. I would have it checked out to make sure nothing is wrong. On all the heat pumps that I ever heard you will hear the reversing valve switch, the outdoor fan stop and the compressor run without the fan running. After the heat pump defrosts the reversing valve swifts, the outdoor fan starts back and most of the time steam comes up from the unit. I would guess that maybe your reversing valve is not shifting all the way over into cooling mode when it goes into defrost. The tech should be able to manually send your heat pump into defrost mode to check things out. Please let me know if you find there is a problem so we can let others know your findings. Sorry that I can not give you any help on this. Steve

      3. Steve,
        Thanks for the info. I am certain our issue is not the unit entering defrost mode; I have heard it go into defrost mode. This noise is different and so loud it wakes us in the middle of the night. Our neighbors have also commented on it.

        I work out of town most of the time and, unfortunately, my wife is left to deal with the warranty service guys. During the first visit, the tech did not even look at the unit; he stood at the front door and told my wife the noise is normal and caused by the unit going into defrost mode. The second, and last, visit the tech did look at the unit but the issue couldn’t be replicated due to warm weather; they told her the noise is normal and caused by the unit going into defrost mode.

        The company tasked with warranty service sends out unprofessional employees who are rude, sexist, and uninterested in helping. I don’t know who else to contact. Do you have any suggestions? I’m thinking about contacting the manufacturer, BBB, and a lawyer.

      4. Hi Kenny! That is terrible that it makes that loud noise! Wow! I think I would try to contact the manufacturer’s local distributor. Most distributors have special techs that take care of problems like you are describing. I hope you get a new unit out of the deal. I know that Bryant, Carrier, Goodman, American Standard, and Trane have distributor specialists that try to solve tuff customer problems. I hope you can get this taken care of soon! If it is below 30 degrees out then you might consider going into “Emergency Heat” mode at night to keep the heat pump from running and waking everyone up. A little tough on the electric bill, but at least you can get some sleep. Sorry that I can not be of much help. Steve

      5. Steve,

        You’ve been a big help. Thanks.

      6. Thanks so much for your kind words, Kenny! If you ever have any questions please let me know. Steve

  8. 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

  9. 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

  10. 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

  11. 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!

  12. 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

  13. 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

  14. 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

  15. 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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