AC problems • Troubleshooting guide
AC Making a Strange Noise? What Different Sounds Can Mean
A new sound from the indoor or outdoor unit is the system telling you something changed. Some sounds can wait; grinding and loud buzzing cannot.
Or dial (213) 214-0220 — available 24/7.
The short answer
Air conditioners make noise, but they make the same noise every day. A sound you have not heard before is worth paying attention to, and the type of sound narrows the cause quickly. Buzzing from the outdoor unit usually means an electrical part is struggling, often a capacitor or contactor. Rattling is loose: a panel screw, debris in the fan, or a fan blade that has shifted. Squealing is a belt or a bearing. Grinding is metal on metal, and that one means switch it off now.
Indoors, a whistle at the return usually means the filter is clogged or the return is undersized, and a clunk when the system stops can be ducts flexing. The rule is simple: soft, occasional new sounds can wait for a scheduled visit; loud, continuous or worsening sounds, and anything that comes with heat or a burning smell, mean turn the system off and call.
Most common causes
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Buzzing at the outdoor unit
A failing capacitor, a contactor that is chattering or stuck, or a compressor that is trying to start and cannot. A steady loud buzz with no fan movement is a stop-and-call sign.
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Rattling or banging
Loose screws on a panel, twigs or debris inside the fan guard, a fan blade out of balance, or a mounting that has worked loose over years of vibration.
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Squealing or screeching
A blower or fan motor bearing running dry, or on older systems a slipping belt. It gets worse with run time and eventually becomes a seized motor.
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Grinding or scraping
Bearings that have failed completely, or a fan blade hitting the housing. Running it further destroys the motor and can damage nearby parts.
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Clicking that repeats
A relay or contactor cycling without the compressor starting, or a thermostat fault. Constant clicking at the outdoor unit means the system is failing to start.
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Hissing or bubbling
Refrigerant escaping from a leak, or an expansion valve issue. A hiss that continues after the system stops is a leak.
Safe checks before you call
None of these involve opening the equipment, touching wiring or bypassing a safety switch. If a check needs a tool or a panel removed, it is not on this list on purpose.
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Locate the sound
Stand near the indoor unit, then the outdoor unit, and note which one is making the noise and whether it happens at start-up, during running or at shutdown.
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Look for debris around the outdoor unit
Leaves, a stray toy or a branch resting against the fan guard cause a lot of rattles. Remove anything you can reach from outside the unit without opening it.
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Check the filter for a whistle
A whistling return grille is usually a clogged filter. Replace it and listen again.
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Record the sound on your phone
Twenty seconds of audio helps the technician arrive with the right parts on the van.
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Switch off for grinding, loud buzzing or metal-on-metal
Set the thermostat to Off. Those sounds get more expensive the longer the system runs.
Stop and call if…
How an AC24Repair technician diagnoses this
The technician listens with the system running through a start, a run and a stop, because each stage implicates different parts. They check the outdoor fan and blower for balance, bearing play and debris, inspect the mountings and panels, and test the capacitor and contactor, since a buzz with a weak start is the most common electrical noise.
For hissing they check for refrigerant leaks with proper detection tools. For squeals they check motor bearings and, on belt-driven blowers, belt condition and tension. Because a worn bearing and a failing motor sound similar but cost very differently, they confirm with measurements rather than by ear alone, then explain the cause and the price before doing anything.
Repairs that may follow the diagnosis
Which one applies, and what it costs, depends on what the technician finds. You hear the price before any work starts.
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Capacitor or contactor replacement
Stops the buzzing and the failed-start clicking by restoring a clean start for the compressor and fan.
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Fan or blower motor service
Replacing a motor with worn bearings, rebalancing or replacing a fan blade, or cleaning a blower wheel that has thrown its balance.
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Securing panels, mounts and ductwork
Tightening loose components and adding vibration isolation where the noise is mechanical rather than electrical.
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Refrigerant leak repair
When hissing turns out to be a leak: locate, repair, evacuate and recharge.
Clear pricing before the work starts.
Diagnostic visit fees depend on the time of day. We publish the confirmed diagnostic fees and explain the repair price before work begins.
Help with this across Los Angeles
Technicians come to homes in the San Fernando Valley, West, Central and Northeast Los Angeles.
AC making noise: questions homeowners ask
My AC has always hummed a little. Is that a problem?
A steady, low hum from the outdoor unit while it runs is normal. What matters is change: louder, new, or accompanied by the fan not turning.
Is a loud start-up thud normal?
A single soft thud at start-up can be normal on some systems. A hard bang, or one that is getting louder, is worth having checked.
The noise is only at night. Why?
At night the neighborhood is quieter, so you notice sounds that were there all day. But bearing squeals also get worse after long run times, so note whether it starts after hours of running.
Will a noisy AC break down completely?
Buzzing and grinding usually precede a failure, sometimes within days. Rattles and whistles rarely cause a breakdown but can hide a part that is loosening. Either way a visit is cheaper than a compressor.
Do you carry the parts for this on the van?
Common parts such as capacitors and contactors are usually on the van. Motors and fan blades depend on the model; the technician will tell you the timing before you approve the repair.
Tell us what’s happening. We’ll take it from there.
Call any time. We schedule a technician within 24 hours and explain the price before repair work starts.
Or dial (213) 214-0220 — available 24/7.