HVAC Noise Diagnosis & Cure
How to Evaluate Air Conditioner / Heat Pump Compressor, Air Handler, Ductwork Noises
NOISES, HVAC SYSTEM DIAGNOSIS -home - CONTENTS: 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. Recordings of HVAC system noises. Categories of sound by HVAC system sound type & what the sound significes.
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Air conditioner or heat pump noise diagnosis & cure:
HVAC systems can produce a stunning range of noises many of which can help lead quickly to the trouble source and thus can help tell us what repair is needed.
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 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.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
See HOW to DETERMINE COOLING CAPACITY of air conditioning equipment if the system seems to be working but is inadequate to cool your building. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
First question in diagnosing HVAC system noises is where are the noises coming from?:
Ductwork noises: sounds originate in the ductwork, or may be conducted via ductwork from another source, including noise sources not necessarily having anything to do with the air conditioning or heat pump equipment itself.
See DUCT SYSTEM NOISES
Air Handler & Ductwork Noise Diagnosis & Cure for HVAC Systems
Air handler / blower indoor cooling unit noises: see air leaks, bubbling, clicking, duct noises, fan noises and vibration dampener noises and the articles on these problems listed just below.
Air leaks in ductwork often make a roaring or hissing sound.
See LEAKY DUCT CONNECTIONS and
see VIBRATION DAMPENERS for a discussion of devices used to isolate noises produced by HVAC air handlers such as air conditioners and furnaces.
Hissing that continues for a time then stops may be due to a refrigerant leak or due to an emergency overpressure relief valve on the refrigerant system.
See HISSING SOUNDS, HVAC from A/C or heat pump system
Buzzing sounds (also crackling) at electrical components anywhere, including compressor relay switches, electrical connections at any component, and quite seriously, at circuit breakers can be an indication of arcing and an electrical problem needing prompt attention.
See BUZZING sounds from A/C or heat pump.
see ELECTRICAL SYSTEM NOISES.
see CHATTERING RELAY CONTROL SWITCHES as buzzing and chattering can be confounded
Also see FAN NOISES, HVAC as ticking and chattering may be traced to a fan problem.
Clicking noises from relays & controls can be heard at either the compressor/condenser or at the indoor air handler unit: A failing or defective thermostat or to a defective control itself can cause relays to click on and off repeatedly .
See CLICKING SOUNDS from air conditioner or heat pump system and
see CONTACTOR RELAY DIAGNOSIS & REPAIR. Thanks to reader Michael Anderson.
Clicking and ticking sounds at HVAC equipment are also traced to a loose or bent fan blade hitting an obstruction.
See FAN NOISES, HVAC
see NOISES, COMPRESSOR CONDENSER [just below] for detailed diagnostic help in finding, evaluating, and fixing noisy compressor units. Humming, rattling, explosion noises, bangs, clanks, hissing, squeaks, humming, squealing noises. Complete diagnostics for the HVAC compressor/condenser/fan unit begin
at COMPRESSOR / CONDENSER REPAIR.
DUCT SYSTEM Noises: hisses, whistles, even roars, and occasionally clunk or clank sounds from expanding or contracting metal ductwork. We may trace these sounds to supply ducts, return ducts, registers, to a bad vibration dampener, or other components.
Animals in the ductwork can make a variety of sounds, depending on the critter: fluttering and clanging (a trapped bird), scratching and gnawing (squirrel, mouse, rat, raccoon), buzzing humming (bees or wasps).
See ANIMAL ENTRY POINTS in BUILDINGS.
Blower assembly and fan noises might be transmitted through the air ducts, such as fan squeaking, squealing, rattling. See FAN NOISES, HVAC
Return air leaks: Leaks around the return air register are common and sometimes noisy, perhaps due to a poorly-fit air filter or gasket.
See RETURN DUCT AIR LEAKS. See Take a look at
see LEAKY DUCT CONNECTIONS.
Supply air leaks: Hisses and whistles in supply air ducts and registers might be present too -
see SUPPLY DUCT AIR LEAKS.
Transmitted duct sounds: Also buzzing, rattling, clanking or other noises originating at the air handler/blower unit may be transmitted into the physical ductwork and thus the building, if the system lacks an adequate vibration dampener -
see VIBRATION DAMPENERS.
Voices in the ductwork? you bet. I still recall the discovery that a girlfriend's parents were monitoring our basement activity when suddenly, out of the air duct by our heads, came her dad's voice: Joanne! Time to come upstairs! HVAC ducts in buildings can transmit sounds between building areas, perhaps more so when air is not moving through the ductwork. So watch out.
FAN & BLOWER Noises on heating and air conditioning systems may include ticking or clicking (fan blade hitting an obstruction), humming (motor troubles), rattling (loose fan pulley), squeaking (dry fan or fan motor bearings) and other sounds.
Aside: humming on oil fired heating equipment may be a normal sound transmitted to the building via oil piping attachment points. A/C refrigerant piping may also transmit equipment noises if it is not properly routed and mounted.
Electric motors (other than HVAC compressor motors) also make humming noises when running or when trying unsuccessfully to start -
see ELECTRIC MOTOR NOISE DIAGNOSIS .
Rattles at HVAC systems: look for loose hardware, loose fan blades, pulleys, or motor mounts; also possibly loose ductwork.
See RATTLING & VIBRATION NOISES from A/C or Heat Pump System.
Roaring sounds at the compressor:
See ROARING NOISE from A/C Compressor/Condenser unit
Rumbling or "singing" or siren whining sounds from a fan motor may be due to a broken internal motor part.
See RUMBLING NOISES A/C compressor/condenser or blower fan motor noise.
Screaming or squeals at the A/C or heat pump compressor -
see COMPRESSOR CONDENSER NOISE or hop over to
any of these HVACR noise source descriptions
SCREAMING A/C compressor/condenser unit sounds - warning!
SILENCE no sound whatever coming from HVAC equipment?
Running water sounds, also described as gurgling sounds at the A/C or heat pump system, often heard at the cooling coil or in refrigerant piping can mean loss of refrigerant, a refrigerant leak, or air/gas contamination in the refrigerant piping system.
How to Diagnose & Fix Noises at the Compressor/Condenser & Condenser Fan Unit
If you hear unusual noise at the air conditioning system, these compressor sounds could indicate the need for a simple service and adjustment requirement.
But air conditioner compressor noises might also mean that you face an upcoming costly
repair involving replacement of the compressor itself. You should have the system checked promptly by an air conditioning service technician.
The following is a list of air conditioning compressor noises and what they may mean about
the operation, condition, and remaining life of the cooling system equipment.
Banging or Clanking air conditioner / heat pump compressor noises, at least the costly ones, are usually due to a loose connecting rod, piston pin, crankshaft, or other internal part.
Since compressors on most
modern commercial and residential air conditioning systems are a sealed unit, the only repair is to replace this (costly) part.
A similar clanking or banging sound can be caused by loose internal mounts. Examples of banging, clanking, rattling noises from A/C and heat pump systems are found
at BANGING A/C or heat pump.
Bubbling, hissing, refrigerant leaks: a catastrophic refrigerant leak on the air conditioner or heat pump high pressure side will make a loud hissing sound - but that sound won't continue for any longer than to dump the refrigerant into the atmosphere (something prohibited).
But a more subtle refrigerant leak that results in low refrigerant can result in refrigerant gas bubbles traveling around in the liquid refrigerant line. You might hear bubbling sounds at the refrigerant piping or see bubbles in the sight glass if your system has one.
See REFRIGERANT LEAK DETECTION.
Buzzing noises at the outside fan motor: sounds like a failing motor; check first for loose wiring, arcing connections, arcing or burning at the fan contactor relay switch. The video shown near the top of this page includes
a VIDEO EXAMPLE of BUZZING NOISESat the fan/compressor outdoor unit.
See BUZZING SOUNDS from A/C or HEAT PUMP
Also see "Chattering" noises below.
Chattering or rattling noise in the A/C or heat pump compressor motor at start-up on some models of HVACR equipment whose compressor uses a compressor crankcase heater element if the equipment is started-up before allowing 24-hours of power-on time first.
Example from Carrier:
"On 24ANA7 models, starting the compressor without a
minimum of 12 hours of crankcase heat prior to initial start--up
may result in a compressor chattering noise and possible damage to
the compressor." Details about HVACR compressor motor crankcase heaters are found
at CRANKCASE HEATERS
Watch out: a chattering electrical relay switch at any electrical equipment or appliance can be caused by a variety of problems such as burned contacts, low voltage, loose wiring, equipment malfunction, and a hard-starting motor. If your air conditioner or heat pump compressor motor is having trouble starting that may show up as a buzzing or chattering relay. I'd turn off the equipment and call for an inspection and repair. A hard-start capacitor might get such a motor going but I suspect it's headed for replacement.
Clanking or rattling air conditioner or heat pump compressor: the compressor motor mounts have failed and the motor is thrashing around inside the hermetically-sealed compressor motor can. It may look normal from outside, but the clanking sound is coming from inside the sealed unit. The compressor should be replaced.
Clicking, rumbling, ticking, rattling, clang and clunk at startup: loose hardware: Air conditioner compressor noise could be due simply to loose hardware such as a loose shipping bolt, tubing, or a broken spring. Does the
noise sound like a metallic rattle?
A visual inspection for loose hardware may help diagnose this condition.
Sometimes a shipping bolt may have been left in the unit and could be the source of a rattle. Loose refrigerant tubing or a bend in
tubing that carries it too close to the air conditioner frame or case can lead to rattling that is easily corrected with a careful
Watch out: Beware of both
the chance of electrical shock and of injury from moving compressor
parts like the cooling fan which can easily chop off a finger. Homeowners should not attempt to open, disassemble, or repair an air conditioner compressor as
special training and safety procedures are needed to avoid injury.
Clicking noises from relays & controls can be heard at either the compressor/condenser or at the indoor air handler unit: A failing or defective thermostat or to a defective control itself can cause relays to click on and off repeatedly or the relay may ultimately buzz -
see CONTACTOR RELAY DIAGNOSIS & REPAIR. Examples of clicking noises from A/C and heat pump systems are found at
our FAQs section below.
Hissing or Screaming A/C compressor, also described as "very loud screaming" by some sources , may be caused by excessive internal pressures - a dangerous condition. Compressors include a high pressure sensor that should shut the system down if internal pressures are approaching a dangerous level. But if the compressor continues to run and is screaming, turn it off immediately as the system is unsafe.
Watch out: turn off electrical power to the screaming A/C Condenser / Compressor unit immediately.
Other more quiet hissing noises from the air conditioner compressor motor may (if the motor is a reciprocating compressor) be due to leaky internal valves.The compressor motor makes a hissing sound all during the time the compressor is running; when the motor stops the hissing dies down fairly quickly as the high side and low side pressures equalize. In this case the condition may not be dangerous, just an inefficient system that is costly to operate.
Humming air conditioning compressor motors, fan motors, other electric motors, particularly if the motor is slow to start or does not start at all, may indicate that the starting capacitor needs replacement.
If the compressor hums and never starts it could also be that the compressor motor has seized. In that case, leave the system turned off and ask for service from a professional.
Humming electric motors:
Note that while many electric motors make a modest and unchanging humming sound when running - a normal condition - any electric motor that is seized or having trouble starting may produce a humming sound at start-up, not just the A/C or heat pump compressor motor.
Examples of more humming noises from A/C and heat pump systems are found at our FAQs section below.
Humming, hard-start compressors, oil leaks: Another source of cooling system compressor noise is a low oil condition in the compressor unit. Low oil may be diagnosed perhaps by observing
evidence of compressor oil leakage on or around the unit. Modern residential air conditioning compressors are usually a hermetically sealed unit; it would be
abnormal to ever see oil loss around this equipment. (But don't mistake spilled oil from lubricating an electric motor or cooling fan
bearing for a refrigerant or compressor oil loss.)
Checking air conditioner oil level: A few residential air conditioning compressors and many commercial systems provide
an OIL SIGHT GLASS to permit a visual check of oil levels. On those units, when the compressor is running the oil level on the sight gauge should read 1/2 to 3/4 full.
If adding oil to a commercial unit, be careful not to add too much. Both too much or too little oil can cause compressor noises. On a residential compressor which is usually a hermetically-sealed motor, it is
not possible to see the oil level nor to add oil.
Squeals, normal A/C compressor: Normal air conditioner compressor squealing: Some compressors emit a high pitched noise during normal operation or just at startup. This noise should be brief and just at start-up, and will probably have always been present on the system if it's normal. Examples of squealing noises from A/C and heat pump systems are found at
NOISES, HVAC SOUND RECORDINGS
Start-up Compressor noises - refrigerant floodback: Air conditioning compressor noise could also be due to refrigerant liquid "floodback" into the compressor crankcase.
If this is the problem the noise will
appear only at the interval of compressor start-up. That's why it's useful for the inspector or service technician to be right at the compressor unit when the
air conditioning system is first turned-on. If this is the problem, a low cost repair might work: a crankcase heater can be installed to address this defect.
Watch out: a burned-out crankcase heater on an air conditioner or heat pump can permit liquid refrigerant to enter the compressor, destroying the motor by liquid slugging.
Ticking or loud clicking noises at the air conditioner or heat pump compressor unit fan: the fan blade may be contacting a bent fan screen or a stick or piece of debris that has fallen into the compressor unit; a loose or worn fan bearing permitting the fan blade to wobble might also cause this ticking or clicking fan noise.
Trumpet or trombone sounds from the compressor motor at shut-down, possibly loud, may be due to leaky internal refrigerant valves; Meenen  asserts this is not a problem as long as a hissing sound is not observed.
Overheating, Blocked Coils, & Air Conditioner or Heat Pump Noise? we have had a field report from a reader who explained that a noisy outdoor compressor unit was, according to his HVAC service technician, traced to a blocked, clogged outdoor condensing coil. We speculate that perhaps the compressor was running hot and that correcting air flow across the condensing coil corrected that condition.
Cooling coils (indoors) or condensing coils (outdoors) can become so blocked that air flow is seriously reduced, possibly also leading to an evaporator or cooling coil icing problem indoors or an overheated, damaged compressor outdoors, or simply loss of cooling capacity of the system.
I have a National comfort Products CPO 2464-B unit which is an air conditioner and gas heating unit combined. The unit is about 5 years old. The air conditioner had become increasingly noisy. I called the party that installed the unit. After spending $90.00 for a service call I was informed the the external coils were dirty, opening up the unit light was not visible thru the coils.
I was advised to use a garden hose and rinse the coils. I did not have that available so I turned the unit on and using a watering can, poured warm soapy water onto the coils. this was done several times. the unit now seems to work properly. I did not see this particular problem described on your website. There may be other means of cleaning the coils but this was all I could think of. - C.A.P., Norristown PA
How to Diagnose Noises at a Split System Air Conditioner / Heat Pump Indoor Wall-Mounted Unit
Fujitsu has offered some helpful advice in troubleshooting noises that may be heard at the indoor wall-mounted air handler of a split system air conditioner or heat pump. What's particularly helpful is that Fujitsu points out that some noises that people worry about are actually normal and not an indication of trouble. Quoting 
During operation and immediately after stopping the unit, the sound of water flowing in the air conditioner's piping may be heard. Also, noise may be particularly noticeable for about 2-3 minutes after starting operation (sound of coolant flowing).
During operation, a slight squeaking sound may be heard. This is the result of minute expansion and contraction of the front cover due to temperature changes.
During Heating operation, a sizzling sound may be heard occasionally. This sound is produced by the Automatic Defrosting operation.
Here are some other noise or sound diagnostic clues for split system air conditioners & heat pumps
Bubbling in the refrigerant piping is discussed
at REFRIGERANT LEAK REPAIR and may indicate a refrigerant leak, though bubbling sounds on split systems at the indoor wall unit may be normal for some products - see our note below about water noises.
Clicking noises from relays & controls can be heard at either the compressor/condenser or at the indoor air handler unit: A failing or defective thermostat or to a defective control itself can cause relays to click on and off repeatedly. - thanks to reader Michael Anderson.
A clicking noise might be traced to a failing electrical control in the air handler or outside at the fan-compressor unit, leading to a control switching on and off too rapidly.
Sizzling noises from a split system air conditioner / heat pump may be heard at the wall mounted unit when the system is in heating mode. 
Squeaking noises from a split system air conditioner / heat pump might be heard coming from the wall mounted unit. According to Fujitsu, This is
the result of minute expansion and contraction of the front cover due
to temperature changes. 
Water noises, a sound like running water may be heard in the refrigerant or condensate piping of a split system air conditioner or heat pump while the equipment is running and/or briefly during unit start-up as well as for a period immediately after the unit shuts off.
While usually we consider the sight or sound of bubbles in the refrigerant piping as an indication of low refrigerant, for some systems this may not be the case and for at least some split system models Fujitsu points out that this sound may be normal. 
However if you hear bubbling in the refrigerant piping and the system is not cooling properly, indeed there may be an improper charge or refrigerant may be leaking. 
Readers: 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
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Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to reader Michael Anderson, 8 May 2009, for discussing clicking sounds coming from air conditioning equipment.
Thanks to Diaz, Domingo I. CIV NAVAIR Bldg.2118, rm. 131: firstname.lastname@example.org
- Ming Diaz, Great Falls, MD for editing help with the text about discharging air conditioning compressor capacitors - 3/07 DF]
Thanks to reader Charles A. Plinton, Norristown, PA, for discussing A/C compressor noise, coil cleaning, and system maintenance - August 2010
 Scott Meenen, G&S Mechanical Services, email: ; web search 5/1/11
 Carrier 24ANA Infinity™ Series Air Conditioners with Puron® Refrigerant, 2 to 5 Nominal Tons (Sizes 24-60) Installation Instructions, Carrier Corporation, [copy on file as 24ana-6si.pdf]
 Troubleshooting Split System A/C or Heat Pump Noises, Fujitsu General America, Inc., 353 Route 46 West, Fairfield, NJ 07004, Tel: (888) 888-3424, Tel-Service hotline: (866) 952-8324, Email: email@example.com, Email service: firstname.lastname@example.org , retrieved 8/30/12, original source: http://www.fujitsugeneral.com/troubleshooting.htm [copy on file as Troubleshooting Fujitsu Ductless Mini-Splits.pdf]
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
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