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What your AC's sounds and smells are telling you

Your system usually warns you before it fails. Here's what common signs tend to mean.

Sounds

Clicking that won't stop at startup: often an electrical part such as a relay or contactor. Have it checked.
Humming but the outdoor fan doesn't spin: commonly a failed capacitor. Turn the system off and call a technician; running it this way can damage the compressor.
Rattling: a loose panel or debris in the outdoor unit. Turn the power off at the disconnect and look for anything caught in the fan.
Squealing or grinding: a motor bearing or belt problem. Turn the system off and call for service.

Musty or "dirty sock" smell: moisture and buildup on the indoor coil or in the drain pan. A cleaning during a tune-up usually fixes it.
Dusty smell the first time the heat runs each fall: normal. It should clear within an hour.
Burning or electrical smell: turn the system off at the thermostat and breaker and call a technician right away.
Rotten egg smell near a gas furnace: possible gas leak. Leave the house and call the gas company from outside.