HVAC Making Humming Noise

Most common humming cause. The motor windings are energized (contactor is closed, sending power) but the failed capacitor can't provide the phase shift needed to start rotation. Motor hums for ~30 seconds, then thermal overload kicks in.

Hum from outdoor unit. Fan blade NOT spinning. Sometimes you can manually spin the fan blade with a long stick (power off!) and it'll start running — confirms capacitor.

The contactor's electromagnetic coil is failing — energized but not strong enough to fully close, or contacts arcing. Produces a buzzing or humming sound, sometimes with intermittent operation.

Hum is at the outdoor unit's electrical panel. Sometimes visible arcing between contactor contacts. Multimeter test of contactor coil for proper resistance.

The 24V transformer at the indoor unit develops a buzz when overloaded or failing. Sometimes audible loud hum on quiet nights.

Locate transformer at the furnace/air handler. Listen for buzz coming directly from it. Often warm to the touch.

Compressor has seized internally or is trying to start against high refrigerant pressure. Hums loudly, may trip breaker.

Loud hum from outdoor unit. Fan may or may not be running. Hum continues 30+ seconds then breaker trips or thermal overload kicks in.

Modern variable-speed and inverter systems can produce a faint refrigerant hiss/hum during operation. This is normal — refrigerant flowing through expansion devices and valves.

Sound is faint, only during operation, no other symptoms. System cools/heats normally. Sound is a 'shhhhh' or soft hum, not a sharp electrical buzz.

Outdoor unit mounted on loose pad or with worn rubber feet vibrates against the surface, creating a humming/buzzing sound that's actually mechanical resonance.

Touch the unit during operation — feel strong vibration? Pad shifts when you push? Vibration isolators worn flat?