Homeowner answer
Why is my AC making a strange noise?
A new AC noise usually points to a specific part. Grinding or squealing often means a motor bearing or fan problem, banging or rattling suggests a loose or broken part, buzzing can mean an electrical fault, and hissing can be a refrigerant or duct leak. Switch off for loud grinding or banging and book a diagnosis.
Call soon
Most noises can wait for a booked visit, but switch the system off right away for loud grinding, banging, screeching or a burning smell. If you see smoke or sparks, leave the home and call 911.
Updated
A new noise is usually one part telling you it is unhappy, and the type of sound points at the part. Two minutes of careful listening — where it comes from and when it happens — tells a technician most of what they need before they even arrive.
What you can safely check
- Pin down where and when
Do this:With the system running, find whether the noise comes from the indoor unit, the outdoor unit or a vent, and note whether it happens at start-up, while running or at shut-off. A short phone recording helps.
You should see:A clear location and timing you can tell the technician — it often lets them arrive with the right part.
When to stop:Stop listening the moment you hear grinding, screeching or banging — go straight to switching the system off.
- Switch it off if it is loud
Do this:For grinding, screeching, banging or any burning smell, set the thermostat to Off and leave it off until the system has been checked.
You should see:The noise stops, and nothing gets the chance to fail harder while you arrange the visit.
When to stop:Stop and get everyone out if you see smoke or sparks — call 911 from outside.
- Reseat the filter and vent covers
Do this:Check that the filter sits fully in its slot — one bent or half-in filter can whistle — and snug any loose vent-cover screws by hand.
You should see:Whistling or rattling at a grille stops once the filter is seated and the cover is snug.
When to stop:Stop at the vent covers — do not open the air handler or its blower compartment for any reason.
- Clear around the outdoor unit
Do this:Walk the outdoor unit and pick up sticks, leaves, palm fronds and toys from around it — from outside the cabinet only.
You should see:Nothing leaning on or sitting in the unit, with a clear foot or two of space all around.
When to stop:Stop at the grille — never poke anything through it or reach inside, even with the system off.
- Listen for a hum that goes nowhere
Do this:With cooling on, listen at the outdoor unit for a hum or buzz with no movement, or for repeated clicking.
You should see:A steady hum with the top fan spinning is healthy.
When to stop:Switch the system off if it hums without starting or clicks over and over — both point to electrical parts, and running it risks the compressor.
Likely causes
- Worn motor bearings
Grinding or squealing from the indoor blower or outdoor fan often means a motor is wearing out.
Handled by a technician
- Loose or damaged fan blade
Banging or rattling can come from a blade striking something or a part that has worked loose.
Handled by a technician
- Failing capacitor or contactor
Loud buzzing, humming or repeated clicking often means an electrical part is failing to start a motor.
Handled by a technician
- Refrigerant leak
A steady hiss or bubbling sound near the lines or coil can be refrigerant escaping.
Handled by a technician
- Loose filter, panel or vent cover
Whistling or rattling at a grille is often a filter out of place or a cover that needs tightening.
Handled by you
Call a pro when…
Book a diagnosis for any new noise that lasts more than a day, and switch off and book right away for grinding, banging, screeching or a burning smell.
- Grinding, screeching or banging from either unit — switch it off first.
- Buzzing, humming or repeated clicking at the outdoor unit, especially if it will not start.
- A steady hiss or bubbling near the lines or the indoor coil.
- Any new noise that lasts more than a day, even a mild one.
- A burning or electrical smell alongside the noise — switch off at the thermostat and the breaker.
- Smoke or sparks — leave the home and call 911.
Next step
Book a diagnosis
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Common questions
Is a clicking noise from my AC normal?
A single click when the system starts or stops is normal; it is a relay or contactor switching. Repeated clicking without the system starting points to an electrical fault.
Why is my AC making a hissing sound?
Hissing can be air escaping from a leaky duct or refrigerant escaping from a leak. A brief hiss at shut-off can be normal pressure equalizing, but a constant hiss should be checked.
Why does my outdoor unit hum but not start?
A hum without the fan or compressor starting often means a failed capacitor or a stuck motor. Switch the system off to protect the compressor and book a diagnosis.