AC Making Loud Noises

AC sounds tell you a lot about what's failing. The same noise — say, a loud hum — can mean three completely different things depending on whether it's coming from the indoor or outdoor unit. Here's a sound-by-sound diagnostic.

Most Likely Causes

Loud BANGING from outdoor unit

Usually a loose or broken compressor mount, or debris (rocks, sticks, ice) hitting the fan blade. Bangs from inside the compressor itself can mean rod failure — that's a compressor replacement.

SCREECHING / metal-on-metal

Failing motor bearings (blower motor or condenser fan motor). The screech gets louder over weeks until the motor seizes. Address it before the motor fails completely — bearings = $80, full motor = $400+.

Loud BUZZING from outdoor unit

Most commonly a failing capacitor or contactor. Sometimes a stuck relay. The unit hums but the fan doesn't spin. Don't let it sit and buzz — it'll burn out the compressor windings.

GRINDING

Motor bearings (same as screeching, but later stage) or a blower wheel that's come loose and is rubbing against the housing.

RATTLING

Loose access panel, debris in the cabinet, or a fan blade that's lost a balance weight. Easy fixes, but the off-balance fan will damage the motor over time.

HISSING from copper line set

Refrigerant leak — usually at a flare fitting or a Schrader valve. Don't ignore. Refrigerant in the air isn't acutely dangerous, but the leak is going to drain your system in days.

GURGLING / bubbling

Refrigerant moving through the line at low pressure (you're low on charge), or air trapped in the system from a recent service that wasn't properly evacuated.

CLICKING repeatedly

Thermostat trying to start the system but the relay or contactor isn't responding. Could be the thermostat itself, the contactor, or low-voltage wiring.

Try These Steps Before You Call

• Identify whether the noise is coming from the indoor unit (air handler) or outdoor unit (condenser).
• Note when it happens — at startup only, continuously, or randomly during operation.
• Take a 30-second video with your phone and send it to us — we can often diagnose from audio alone.
• Don't ignore screeching or buzzing. Both indicate components that will fail completely soon.

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