Photograph of  This new compressor was placed directly against
a brick wall. One third of its condenser coil cooling ability was blocked. HVAC Noise Diagnosis & Cure FAQs

  • HVAC SYSTEM NOISE DIAGNOSIS FAQs - CONTENTS :frequently asked questions about how to diagnose & repair noises or sounds traced to HVAC systems including compressor/condenser units, air handler units, air conditioners, heat pumps, furnaces, ductwork, motors & controls.
  • POST a QUESTION or READ FAQs about air conditioning or heat pump system equipment or control or ductwork noises: cause, significance, cure
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Air conditioner or heat pump noise diagnosis & cure questions & answers:

FAQs about the causes & cures for air conditioner, heat pump, or other refrigeration system noises help tell us what repair is needed.

This air conditioning repair article series 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 shrieking and howling or death-rattles that sometimes (not always) presage costly compressor damage indicating air conditioning compressor or A/C compressors at or near end of their life.

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A/C & Heat Pump System Noise Diagnosis & Repair Noise FAQs

Photograph of  this ugly duct routing risking water entry, mold, rodents, high operating cost.These questions on finding and curing noises traced to heating, ventilation, or air conditioning systems were posted originally at NOISES, HVAC SOUND DESCRIPTIONS

Question: put the A/C compressor under ground to suppress HVAC noise?

(June 17, 2015) Andy Gough said:
Would an in ground compressor installation be an effective way to suppress noise?


Perhaps it would, Andy, but on the other hand the cost of an underground HVAC compressor installation would be quite high as would be service and maintenance of such an installation.

As the compressor-condenser requires air movement across the condensing coil to operate the design would also still have to have openings to the atmosphere. In all I'm doubtful that such an approach is feasible.

Question: diagnose intermittent noises from an A/C unit

(Aug 26, 2015) Susan said:
I live in a townhouse with 8 units side by side. There is an intermittent noise which I am sure is my neighbors roof unit, because I have turned off my unit, and the neighbor on the other side has tested hers.

When I had mine checked for noise by the AC company, the repairman said it was working fine (great) and that the noise is normal (really?)
My neighbors with the noisy unit said they would have it serviced. The noise is loudest when I'm in bed on the 2nd floor, and almost goes away when I stand up.

Is there a way to insulate these roof units? Or my ceiling and walls? There is no attic, ductwork is between 1st and 2nd floor.
Thanks for all the information you provide.



First we need a diagnosis of the noise. For example if a motor or fan bearing is failing or there are loose components on the AC unit, repair is required and is straightforward.

Other "intermittent" A/C noises may be a motor trying to start, unsuccessfully.

If the HVAC tech says the particular system you describe are operating normally then discuss sound isolation with the HVAC service tech. For example, besides fixing mechanical problems, there are sound isolating pads that can go beneath rooftop equipment - though lifting it to add pads can be an expense.

Also see the noise and sound isolation article series beginning at SOUND CONTROL in BUILDINGS

Question: heat pump buzzing due to motor mounts?

(Nov 29, 2015) Margaret said:
The heat pump, outside unit, is mounted on steel brackets. It buzzes constantly when running. Could this have anything to do with it's mount? Also, right before it cuts off, it makes a louder buzzing noise and then stops. The noise is affecting my life! It makes me feel ill due to the vibration.


Yes loose mounts can be a source of buzzing.

Question: copper line "exploded" on my heat pump

(Jan 1, 2016) Rich said:
Had a copper line "explode" on my heat pump last night. Sounded like a cannon going off. What might have caused this and what is the fix?



I trust you have turned OFF the system completely, both for safety and to avoid further damage.

If I assume that you are referring to a high pressure-side refigerant piping line that has burst, I suspect that a refrigerant metering device such as a TEV (thermostatic expansion valve) became jammed or clogged, the compressor kept pumping away, an excessively high head pressure developed, and the particular system did not include a high pressure limit switch (as is often not present on residential systems).

Question: AC makes noise when switching on and off automatically - like a table saw

(Jan 11, 2016) Bree said:
One of my air conditioning units (the downstairs unit) has begun making a very strange noise (at least I think so) recently. It doesn't make the noise when I switch the unit on and off manually, but rather when it's running and automatically stops and starts. When it stops, it makes a sound that I can only describe as sounding like a table saw. I had a technician out today, and he said that it's just the A/C unit switching over to defrost. I'm concerned because of the fact that I don't hear any other units in my immediate vicinity make the same noise.It also makes a strange noise when it starts back up again. Does anybody know what that noise could be?


Check the buzzing sound descriptions; I suspect a failing relay.

Question: heating units make popping noise at startup

(Jan 25, 2016) Michelle said:
For the past three weeks my heating units make popping noises (inside the attic) when it first comes on and for like 5-10 minutes afterwards. The first week, I heard a louder sound outside at start up (that stopped). The noise is annoying and interrupts my sleep. I keep the thermostat on 68 (more so for the pipes) we're in the 30s or below freezing. Before I call out an technician, I would like to have an idea of what my problem maybe. I read the "ductwork noise" section, but still confused. Please help.


See HVAC NOISE-4 POP, RATTLE, ROAR, RUMBLE - loud start, popping, rattling, roaring, rumbling, running water

Question: Lennox 3 & 4 ton XC16 with SL280UHV, SACH80V blowing warm air, straining noise at startup

(Apr 9, 2016) Ron said:
I live in the desert area community in southern CA and have a 5 yr old Lennox 3 & 4 ton XC16 with SL280UHV, SACH80V & ADP Coils on each. Last March the 4 ton unit started to blow warm air. The tech said refrg was low and added 4 lbs and system worked good after that.

March of this year same 4 ton unit felt liked it was blowing cool air but couldn't keep the temp we set inside the house.

Now I'm told there may be a slow leak in the line but they cant find any using bubble spray or by electronic sniffer. tech said the lines were frozen causing a loud straining noise at the condenser so I needed a kick starter since the system was too much amps a start up!

Qusetions are

1) if the lines are frozen and have very low refrig how can you find the leak?

2)shouldn't the refrig be topped off and then tested for noisey start up etc and leakage before now suggesting a Isolation test on lines? thanks


The leak should be found and fixed; but I worry too that the compressor motor may be failing.

Adding 4 lbs of refrigerant is no "small leak" it's a catastrophe: dumping harmful refrigerant into the environment, dumping your money on the ground. I know that it can be hard to find and fix a leak (having made a market for "stop leak" additives that are somewhat controversial), but geez: converting the HVACR repair business into a delivery route for refrigerants is not so great a business plan and not too great for customers.

That griping said,

- icing can be caused by low refrigerant, until the refrigerant gets so low that there is no cooling at all

- yes icing up a thermostatic expansion valve can cause trouble for the compressor
A compressor can have trouble starting up against high head pressure that can occur if the TEV is not passing refrigerant out into the low pressure side of the system.

Leaks are found by inspection and testing.

Don't keep topping off refrigeant.

The extent of compressor damage can also be assessed by looking at its current draw.

Question: inside unit vibrating noise

(May 3, 2016) Aj said:

Inside unite was making a vibrating noise. Condensation around the base of the unit. Found the line and outside unit frozen.

Took out filter, turned on fan and let it thaw. I turned on the unit to AC this morning. It is still making the vibration noise but pushes cold air. Ideas?


Start by looking close for loose motor mounts, door, or other mechanical components. Also have your HVAC tech check for a bad TEV.

Question: Air buffeting noise problems in HVAC systems: Cause & Cure

(May 27, 2016) Steve said:
Installed replacement air handler in Nov. Was fine until late spring when I began installing factory supplied a/c components. Upon installing coil shelf kit, there was unacceptable "air buffeting" @ air handler, even w/o the coil in place

.Needed a/c ,so set coil, had it hooked up.Cools fine, but have unacceptable buffeting. Any suggestions?


Steve, a "Buffeting" effect and noise can occur in HVAC systems where a variable speed fan is installed, occurring when the fan turns on.

Under-sized ducts or too much flexing in ducts, or too-loose flexduct runs can add a "spring-like" effect to the duct system that can make the buffeting problem worse.

I've also read that a too-dramatic transition between duct sizes or diameters can create an obstruction that contributes to HVAC air buffeting problems as will properly matching plenum shapes to duct shapes.

For example installing a square box elbow between two round ducts can cause buffeting.

Those are some things for which to check.

Some HVAC techs apply a "band-aid" by adding an adjustable damper in the duct system but I agree with critics who say that's not finding / fixing the problem.

If the fan speed is indeed variable, try setting it to a lower speed.

Question: Vibration noises at scroll / rotary HVACR compressors

2016/06/04 Jim said:
I have a new Goodman, 14 SEER, R22 scroll-compressor, Split-System HVAC unit that was purchased/installed in 2009 but not put into service and started until today. The unit cools my home very well however the condensing unit is very noisy and has a great deal of vibration in the low side refrigerant line and the condensing coil is physical moving about a lot inside the housing in perfect time with the vibration.

The ambient temperature was/is 105F with the HS press @ 250psi and the LS Press @ 78psi which yields an evaporator coil temperature of around 43F.

The pressure readings are stable with no fluctuation what-so-ever. I am a certified tech with over 25 years of experience and I have never seen a unit do this before now. Anyone have any experience with this problem and/or any advice? Thanks a bunch, Jim

Reply: studies report analysis of vibration in rotary compressors or scroll HVACR compressors


Look first for loose mounting bolts or screws anywhere on or in the compressor/condenser unit; also look for refrigerant tubing left un-insulated and in contact with a vibrating service. You probably already know those common noise sources and have perhaps more field experience than I do, so let's think and argue about this a bit.

Only a mechanical movement can cause vibration; we can probably exclude outside influences such as wind or nearby mechanicals on other systems;

Is the pad level and secure?
Can you take a mechanic's stethoscope and check components for vibration: seems to me we have either the compressor motor or the fan motor. I don't think that even a bad control such as a relay or TEV would vibrate at such a high frequency as to cause the noise you describe.

I'm speculating you'll trace the noise to the compressor motor itself: perhaps a sticking valve. I did some research on vibration in rotary compressors: you'll want to use Google Scholar to search for and find these articles:


The presence of these and other research papers to me as a non-engineer, suggest that vibration in rotary or scroll HVACR compressors is a known and studied problem.

For a catalog of different sorts of HVAC noises and what they may mean, see NOISES, HVAC SOUND DESCRIPTIONS - Five groups of HVAC sounds, listed alphabetically from Banging to WuWuWu help track down the noise source & suggest repairs.

Question: outside air conditioner or heat pump is making funny sounds

(Mar 7, 2015) jr said:
my outside heat pump is buzzing and the fan is not running

(June 6, 2015) Dale said:
HVAC contactor does loud short buzz about every minute. I heard this yesterday and replaced the contactor and it is still happening. Could this involve the 24VAC from the thermostat, or capacitor, or something else?

(July 8, 2015) Nelson said:
A/C is turned off and if I turn power on outside its a whining or buzzing sound in the house. the a/c is not on inside or turned on. but the sound is coming from the air handler area. have to turn power off out side to a/c to stop noise.

(July 28, 2015) Benny said:
A month ago I replaced the condenser fan motor & capacitor. Now the entire outside unit is shaking so bad when the fan is running that it cracked the cage. And, it is making alot of noise & not cooling again. Any idea what it may be?



Bent fan blades,
Worn or damaged fan bearings
Bent fan drive shaft
Loose fan mounts

Do not keep running this unit; it will be further damaged and it is unsafe.

Question: air conditioner making noises like a wood chipper

(Aug 10, 2015) dave said:
my unit was making noises like a wood chipper and it wasn't coming from the fan blades or motor.

i checked around the unit and the pushed big hose (electrical wiring) that connects to the condenser that goes into the circuit breaker. it started to spark inside the unit…

i immediately shut the ac off and pushed the wires into the unit more and the noise stopped afterwards. my question is do i need a technician to come out and see if there's a wiring problem or do you think the problem is fixed? it seems to be running fine now…

but i'm worried that there's some miswiring inside somewhere.



Well I'm glad you shut off the sparking electrical components.

But just pushing wires into a unit is not a safe repair. Electrical wires that are damaged or not secured against movement can be expected to move again, short, and possibly start a fire or electrocute someone. So yes my opinion is that a repair is needed. I'd shut down the equipment until it's fixed safely.

Question: periodic BANG from my air conditioner keeps me from sleeping

Aug 26, 2015) Noise in HVAC unit said:

(Aug 26, 2015) said:
sorry, didn't add my email
Hi all, I'm seeking professional help in desperation. I haven't slept for weeks because of odd HVAC noise in my apartment.

I live in a rental unit in NYC, as of March. The HVAC unit had been functioning perfectly with normal sounds using heat in the winter and a/c in the summer. Then suddenly around 8 weeks ago I started hearing a periodic BANG from the unit. It happens every 10-12 minutes, about 5 times an hour.

It's a very loud vibrating BANG, sounding like someone is hitting metal with their hand. I initially thought there was something wrong with my unit, but I tested it and found that even with my unit turned absolutely off, the sound would continue.

After much monitoring, I've found it occurs only in the wee hours (4, 5 am, continuining through to 7 or 8 am, possibly later although I leave for work.) It never occurs when I come home, when I turn on the A/C, for example (another argument it's not coming from my unit). It always begins and wakes me around 4 in the morning (hence the lack of sleep), and the cyclical nature is like psychological torture.

My building has not been able to track the source, and I'm thinking it must be the person above who keeps late hours, who comes in late and then maybe turns their unit on and eventually it causes some kind of popping in my unit below. I asked them to bring in a professional to resolve this as this is causing me emotional and physical strain.

Does anybody recognize what this sound might be, and what might be causing it? Thank you in advance.

Reply: banging that is not coming from the HVAC system? occurs at regular intervals.


From just your e-text I don't think we can make a reliable diagnosis of an HVAC noise source, though this article series including NOISES, HVAC SOUND DESCRIPTIONS does describe some sources of banging sounds in A/C equipment. Don't forget to consider other noise sources such as banging water pipes.

Also expanding or contracting metal ductwork can make similar sounds.

Also see

HVAC NOISE-1 BANG BUZZ VIBRATE - banging, bearings, buzzing

BANGING A/C or heat pump

Question: vibrating roaring sound, then loud rattling knocking sound

(Sept 14, 2015) Anonymous said:
My air when it comes on it makes a sound like a vibrating sound roar then it stops

(Sept 26, 2015) Kelly said:

The air handler in my FL townhouse is making a loud rattling/knocking sound for 3 or 4 seconds about every 3rd or fourth time the compressor kicks in. The A/H sits right outside the bedroom door, and it makes sleeping difficult.

The A/C tech says it is refrigerant surge and is caused by a bad install design. The compressor sits in a platform on the ground level and the air handler is on the third floor. (bottom floor is garage.) Refrigerant lines go up into the attic and down to the air handler.

Tech says even a whole new system may not fix it as the units aren't designed to push the liquid up 3 floors. There are over 100 units in the complex, but I haven't heard anyone else having this issue. Any solution?


I would ask for help from a senior HVAC technician on-site. You've answered the question yourself: if it were a design issue then every installation of the same design at your townhouses would behave the same way.

Question: heat pump makes a metallic noise

(Sept 28, 2015) Laura said:
The heat pump makes a metallic noise constantly, even if outside is 16 gr C. I am sure it is not ice. On the contrary, when the A/C is on, the metallic noise disappear. The heat pump is now 3 yrs and it was noisy from the beginning.Does anyone have any idea? Thank you so much!


Laura, see RATTLING & vibration noises from A/C or Heat Pump System

Laura: Your service tech may want to check the reversing valve.

Question: tapping sound in wall where refrigrant piping runs

(Feb 14, 2016) Don said:
I have 10 year old heat pump where the interior unit is located in the attic, the exterior near the house, and the coolant pipe is run through the exterior wall of the house to connect the two units. Over the past couple of years there has been a "tapping" sound in the wall where the coolant pipe is located. That tapping does not occur when the unit is running and stops about 5 minutes after I turn the unit off. I have checked the condensation pipe and it is not that. Any ideas?


Could we be hearing movement of the piping through a tight opening or across a surface due to thermal expansion / contraction as the refrigerant piping temperatures change?

Question: American Standard unit Clicking noise

(Aug 6, 2016) Kelly said:
My American Standard unit outside is clicking, it turns on, Clicks turns right back off or on. It does this for 2-4 times then it will run for a couple minutes. Then it repeats the cycle. I have it onow video any suggestions?


Kelly please see CLICKING SOUNDS from air conditioner or heat pump system, then see our discussion of chattering relay switches and controls at RELAY CONTROL SWITCHES and let me know how that works for you and your service technician.

Question: loud blower unit, loud snapping and clicking noise

(Sept 5, 2016) Anonymous said:
I am in a brand new build apartment with a Goodman air conditioning unit. The blower is extremely loud, and there is an extremely loud snapping and clicking that happens whenever the unit kicks in. The sounds are coming from just behind the filter in the main vent to the living room.

I have had the guy who installs the units look at it and told me it was the condenser coil making the noise and there is nothing he could do about it. Other apartments have also complained. The guy across the hall has to sleep with the bedroom door closed.

I have not gotten a single night's sleep since I moved in here due to this noise. What can I tell them needs to be done to correct the noise. If I had this installed at my house, I would have had it ripped out the next day. I haven't read anything yet that seems to be something that I can take to this guy that denies that the noise can be fixed. Thanks in advance for any help you folks can give!


Anon there are two different concerns that occur to me:

1. The blower speed and sound and air velocity vs. duct design, insulation, requirements, and perhaps soundproofing between you and the AC unit

2. Loud clicking and snapping that you hear in, at or near the air handler or through nearby HVAC ducts, especially when the blower unit first starts are often traced to a very excited electrostatic air cleaner who is zapping dust particles loosened when the air flow starts up.

Question: loud banging noises coming from our A/C unit and constant cut off of the air supply

Sept 7, 2016) Jason B. said:

Hello....I am frustrated as well with loud banging noises coming from our A/C unit and constant cut off of the air supply through the vents while the outside fan spins aimlessly. We have had 2 service calls with 2 different technicians only to be told they could not tell the problem but charged us anyway for each service visit. I even went so far as to record the banging sound on my phone. Please help!! my email is


I'd give the service manager a call and ask for a more-experienced technician.

Also be more specific about where the noise is occurring: outdoor unit, indoor air handler, ductwork.

(Sept 7, 2016) Anonymous said:
... thanks for that response. the noise is coming from directly inside the main vent in the living room. That vent is located a few feet from the indoor unit located in the ceiling of the bathroom.

I have had 2 men look at the unit and still tell me and other tenants that the noise is the condenser coil making the noise and it only happens when the unit kicks on. That is true, but it is so loud that it can wake me up out of a deep sleep. I can't believe that that is acceptable. Why would the condenser coil make that noise, and is it possible for the condenser coil to do that? I'm at a total loss.

Thanks for any further information you and/or others might be able to offer.

Reply: give the service manager a call at your HVACR repair company; politely explain the problem and ask for help from an experienced service technician.

Question: vibration noise after fan replaced

2016/09/15 Steve said:

Hi, I had my fan replaced on my outside condenser , heat pump. Now, there is more noise and vibration than before, I had to put foam between top grill and frame. It is a 4 ton unit and he put in a 1/2 HP 1075 rpm motor. He said it is because my last motor was a piece of crap. Does this sound like the right size motor. thanks

Previous question error. He put in a 1/3 hp motor, not 1/2 hp, thanks


Perhaps more noise and vibration from improper mounting or an unbalanced fan blade set? Look for loose parts first.

Unless the fan is running at the wrong speed, I would not expect fan vibration and noise to be due to motor size.


Continue reading at NOISES, HVAC SYSTEM DIAGNOSIS - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see this

Index to Text Descriptions of A/C or heat pump sounds & possible causes / cures

Common HVAC Failures that Cause Noises

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