Photograph of an old burned-out air conditioning compressor unit HVAC Noise Descriptions & Recordings - Group 4
Air Conditioner / Heat Pump Compressor, Air Handler, Ductwork Noises

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How to diagnose loud start, popping, rattling, rumbling, running water sounds or just plain silence at an air conditioner or heat pump.

Air conditioner or heat pump noise diagnosis & cure: this air conditioning repair article discusses the diagnosis and repair of air conditioning compressor noises which range in importance from normal (if annoying squeaks and squeals, to rattling loose bolts and hardware, to costly compressor damage indicating air conditioning compressor or A/C compressors at or near end of their life.

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HVAC Noise Group 4: loud start, popping, rattling, rumbling, running water sounds at an air conditioner or heat pump. Also "no noise" or "silent" equipment

Article Contents

Loud Noises at Startup, then Quiet Compressor

I had problems with my central unit freezing up and dripping water on my fan motor. I Decided to have a technician change out my coil inside. When he left my central unit outside starting making noises I never heard before.

My air conditioner kicks on inside first then after a couple of minutes it tries to kick on outside , it makes a loud noise then it runs finally and quiet like a central unit should run outside.

What is causing this all of a sudden? We never had problems until he changed my coils out..not calling him back to my house. - Bruce McCann

Reply: Indoor coil freeze up vs outdoor sounds??

I am not clear on why a coil freeze-up indoors would lead to changing the outside coil - maybe low refrigerant or improper high side vs low side pressures? In any case, I would indeed give the service tech a call, tell him or her what you are observing, and give the technician an opportunity to return, see what's wrong, and fix it. Most technicians want their work to be correct and successful. Let us know what you're told - it will surely help other readers.

Air Conditioner or Heat Pump Popping or "popcorn" sounds - Warning!

Question: a/c condenser unit sounds like a popcorn machine only when going off

Question: I just had a new compressor installed outside two weeks ago. In the last few day this unit has emitted loud intermittent "popping" sounds like firecrackers that keep us and the neighbors from sleeping! Cannot see anything obvious and not sure if related to recent repair. It is not a regular pattern or a metallic sound. Recorded for our a/c guy and he thinks a wire is loose and the sound is an electrical contact sound. Ideas? Thanks. Mike C. 9/12/11


Watch out: an A/C or heat pump compressor motor that is receiving liquid refrigerant on the suction or intake side of the system can make a popping noise or popcorn noise - and is at serious risk of becoming damaged or destroyed.

Liquid refrigerant should be found only on the output side of the compressor/condenser unit not at the compressor intake.

Causes of "liquid flooding" of an HVACR compressor include improper refrigerant charge (too much refrigerant) or a malfunctioning refrigerant metering device at the cooling coil.

You should shut off the system immediately and leave it off until an experienced HVAC service tech has diagnosed the problem, hoping that we might avoid destroying the compressor motor.

Other causes of popping or knocking sounds in air conditioners or heat pumps - knocking sounds can mean a bad motor bearing, bad motor mount, or on occasion, a spinning fan blade hitting a component in either the air handler or outside in the compressor/condenser unit.

In automotive A/C systems, condensate dripping onto a hot exhaust manifold can also cause a "popping" sound in some designs.

Watch out: electrical shorts and loose wires can also make a "popping" sound at any electrical equipment. Turn off the equipment to reduce the risk of a fire or dangerous electrical shock.

Any readers with other suggestions please post a "popcorn noise" comment.

A/C or Heat Pump Compressor Rattling & Vibration Noises

Question: buzzing vibration sound at the outside compressor/condenser

We just noticed our outdoor compressor unit running noisier than usual. Not exactly a rattling noise but a louder than usual buzzing type sound. It was constant. So we turned off the AC at the thermostat. and looked around the unit to see if we could see anything unusual.

Didn't really know what we were looking for, but didn't see anything unusual.

About 15 minutes later we turned the AC back on and it took about 10 minutes to "kick in" at that time there was only a low humming sound from the compressor. The fan never started. We let it run for about 15 minutes and there was no change.

Just prior to noticing the noise in the first place, we realized that it seemed hotter than it should be in the house and the air blowing out of the registers was room temp, not cool. - Kim

Well, I read the story below about the person who was told to rinse off their external coils. so we gave that a try while the AC was turned on.

After about 3 minutes of spraying with the garden hose it started up. It sounded smooth, too, no strange noise, except for the water spraying off the fan blades. YAY!

Well Darn it. When we woke up it wasn't running again. I got it started again by spraying with the hose. But the whole system doesn't seem to be blowing as cold as it used to. And my guess is on the next cycle the compressor wont start on it's own again. - Kim

Reply: common sources of HVAC rattling noises

Rattles at HVAC systems: look for loose hardware, loose fan blades, pulleys, or motor mounts; also possibly loose ductwork.

We have seen rattling traced to loose motor mounts, loose motor bearings, loose equipment covers, refrigerant piping vibrating against a surface, and loose fan blades impacting or ticking against other equipment components or surfaces.

Watch out: a sort of rattling sound may be made by an air conditioner or heat pump (or other HVACR equipment) compressor motor if the refrigerant level is too high - that is if the system has been overcharged. This condition can damage or even destroy the compressor very quickly.


Also see BUZZING sounds from A/C or heat pump

Question: blown compressor in Costa Rica, swapping in parts, rattles & vibrations

(Feb 20, 2014) Greg said:

I have a Parker 9,000 BTU split unit with a blown compressor. A "tech" here in Costa Rica swapped out blown compressor or a newer, undamaged compressor from a 12,000 btu floor unit. He also swapped out the capacitor, sizing the new capacitor correctly to the new compressor.

The a/c starts, blows cold but consistently rattles/vibrates loudly after a half hour and then completely shuts down due to excessive heat at the outdoor unit. Any ideas as to what the problem may be? Help please!


Hi Greg.

SHUT DOWN the system immediately then come back to read more.

Now, take a look at noise group 4 in the "More Reading" links just above.

I wanted you to turn off the unit because if it is suffering from an over-charge or excess refrigerant, or if the refrigerant metering device is not working properly the liquid refrigerant can cause "liquid slugging" which basically destroys the compressor.

With that concern in mind you want to be sure that the right quantity of refrigerant (and correct type) was installed.

While you're at it check for loose mounting bolts too, but I'm guessing its an improper charge or a problem with the refrigerant TEV (thermal expansion valve) metering system, or something else that we're not smart enough to guess at by text-comments.

Keep me posted


Reader follow-up:

Thanks Daniel, you are the man. Now I have some info to go with.

Running Water Sounds in Refrigerant Piping

Our discussion of refrigerant floodback noises - running water or gurgling sounds in the refrigeration piping - has moved to a separate article.

Please see REFRIGERANT PIPING GURGLING or running water sounds in refrigerant piping - floodback or condensate?

Roaring Sounds at the HVAC Compressor/Condenser or other HVAC components

Question: terrible roaring noise

(July 2, 2014) Sandra said:

Roaring noise is Terrible! We have a small condo with one main return duct. The unit sits in an exterior closet room off patio, but abuts the wall that the return vent is in, and there is not much room between the unit and the wall vent.
Is there ANY POSSIBLE way to muffle this loud roar to a more tolerable level?

It's SO loud you must turn the tv up extremely loud just to hear what's being said, and then it's crazy loud when the system rotates to "off" when temp reached inside. You can't carry on a normal conversation when the system turns on, winter or summer, AC or heater!
HELP - please?!

Sandra in NJ



Duct insulation, duct isolation mounts, air handler isolation mounts can all reduce HVAC noise transmission, but for the unusually loud noise you describe I would ask FIRST for an evaluation by an experienced HVAC repair technician, as it's possible that your equipment is not working properly, possibly not safely.

Question: Arcoaire compressor/condenser unit sounds like a lawn mower - roaring?

(Sept 18, 2015) Doll said:
My outside Arcoaire heat pump compressor is making a very loud noise when it comes on. It sounds like a lawn mower or 2.


I would leave the system off and call for repair: it sounds like a failing compressor.

Rumbling Sounds at the A/C Compressor/Condenser or Air Handler

Question: York AC Unit making a rumbling sound in the air handler - sounds like thunder

(July 5, 2014) Ginger Vela said:

Hi Dan, I have a York 19 SEER AC unit that is very efficient, but it developed a rumbling sound from the air handler like low, rumbling thunder when it comes on. At first it was slight but has grown longer over time. It continues for anywhere between 15-30 + seconds and then goes away.

A tech came out to do regular service and, wouldn't you know, it didn't act up, so he did not hear it. It seems to do it less when I have it set colder than I like it.

Since thunder comes from cold air meeting up with warm, I thought maybe that had something to do with it. Probably crazy. Please help so I can share your thoughts with the next tech I might call. Otherwise, it is running well. Bought it in 2009, and it had a defective compressor which had to be replaced in less than 2 years from purchase.

Only I was able to hear the noise of that compressor dying. Thank you.



Interesting problem: rumbling noises from an air handler.

I'd start by looking with care for loose components: loose blower mounts, motor mounts, ductwork, vibration dampener.
Don't forget to check for loose blower fan assembly itself.

FYI, the cause of real thunder, in the outdoor air, is not quite cold air meeting warm. More accurately, the lightning bolt very rapidly heats a small column of air to very high temperatures. The thunder you hear is the noise of the rapidly expanding air.

Horizontal and forked lightning may make a longer rumbling sound.

Reader follow-up:

Hi Dan, Thank you so much for your prompt and helpful suggestions. I'll be able to speak intelligently about possible causes with a technician. My inclination has been to wait for it to get worse only because when the compressor died, no one could hear the agitator-like sounds that I did before its last gasp.

Hopefully, it won't get any worse or louder. Thanks again, Ginger Vela

Hi Dan. I'll be sure to let you and the readers know whenever I get an answer to the rumbling in my air handler. It has been getting gradually worse, but I've not yet called in the technician again since the unit works so well otherwise and is very efficient. Thanks again, Ginger

Also see SINGING, rumbling, whining A/C compressor/condenser or blower fan motor noise as some people combine rumbling and singing in these HVAC noise descriptions.

Vibration & Vibrating sounds at HVAC Compressor/Condenser

Repetitive or Cyclic Vibration Sounds

AC has an intermittent loud vibration that lasts about 15 seconds and goes away.

It occurs about every 30 seconds or so. We just had to have freon installed because the unit was running but never able to achieve the set point. We keep it at 76, so it wasn't like we were straining it. My husband is checking to see if it's level right now since we read a comment that out of level can lead to vibration and from there to a refrigerant leak. We obviously had a leak since the gas was at less than half of its required level. - Rebecca

Reply: Cycling or repetitive noises in HVAC compressors

Rebecca: there are several failures that can cause a frequent cycling noise in HVAC compressors such as

- a start/capacitor that is not working or not able to start the motor - the motor may attempt to restart repeatedly until a hard failure occurs

- a thermostatic expansion valve that is improperly adjusted or stuck open can cause a hunting or seeking cycle in which refrigerant is not only improperly metered, but liquid refrigerant can enter and destroy the compressor

I tend not to suspect a thermal overload because usually when electric motors overheat and trip off on thermal overload it takes much longer than 30 seconds for the control to automatically reset.


Continue reading at HVAC NOISE-5 SCREAM SING TRUMPET WHINE WUWU - scream, sing, squeal, trumpet, water gurgling, whine, whistle, wuwuwu or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see SOUNDS of RUNNING / DRIPPING WATER in DRAINS - for running water sounds traced to a building plumbing drain system

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HVAC NOISE-4 POP, RATTLE, ROAR, RUMBLE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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