The sound started at 2 a.m. in Nelmar Terrace: what each air conditioner noise means, sorted by when you hear it

Two in the morning, a bungalow, a new sound

The house is a 1920s Craftsman in Nelmar Terrace with the air handler in a hall closet, three feet from the bedroom door. For ten summers it has been a background hum. Tonight, at the start of a cycle, there is a metallic rattle, then a rising screech that settles into a whine, and the homeowner is awake with a flashlight. The question they have is “what is that?” The more useful question is “when does it happen?” A sound at startup, a sound while running, a sound at shutdown and a sound only at night point to different parts.

Noises at startup

A click, then a hum, then nothing from outside. The contactor pulled in but the compressor or fan did not start: a capacitor, most often. Shut it off.

A hard thud or bang from the outdoor unit. The compressor starting against pressure, sometimes a sign of a weak start capacitor, sometimes a compressor with worn internals. Not an emergency once; a pattern worth a visit.

A rattle from the air handler that fades as the blower comes up to speed. A blower wheel loose on its shaft, a panel screw gone, or debris in the wheel. In a closet install it is loud; in an attic you may never hear it.

A buzz at the thermostat or the air handler when the heat is called. A transformer or a relay beginning to fail.

Noises while it runs

A steady screech or whine from the air handler. Blower motor bearings. The motor will fail; the only question is when. In August that is a bad gamble.

A grinding from the outdoor unit. Fan motor bearings, or the fan blade touching the guard after a bump.

A hiss or gurgle near the indoor coil. A refrigerant leak, or in the quieter case, a drain trap bubbling. One is a repair; the other is a vent.

A rattle from the outdoor cabinet. Loose panels, which at Vilano and Crescent Beach usually means corroded screws.

A whistle from registers or the return grille. Airflow restriction: a loaded filter, closed registers, an undersized return in a 1950s Davis Shores closet.

Noises at shutdown and in the quiet after

A thump as the fan stops. Ductwork relaxing. Common in attic installs with long flex runs.

A tick-tick-tick from the outdoor unit after it stops. Metal cooling and contracting. Normal.

A gurgle or drip from the air handler for a minute after. The drain line emptying. Normal, unless it is followed by a puddle.

A click every few seconds from the air handler. A relay chattering on low voltage. Not normal.

Tell us when the sound happens

Noises only at night or only after rain

Night is quieter, so you hear what was always there, but night also brings the heat strips in winter and the longest run cycles in summer, so some noises genuinely are nocturnal. A buzz only on cold mornings is the heat strip sequencer. A squeal only on the longest afternoon runs is a bearing that is heat-sensitive. A rattle only after rain is water in the outdoor fan guard or a float switch cycling the system on and off as a clogged drain fills and empties. Neighbors in the National Register districts, where condensers sit close to bedroom windows, notice a change in the outdoor unit before the owner does; take the comment seriously.

Which of these means call tonight

Humming outside with the fan stopped: shut it off and call. Grinding or screeching from any motor: shut it off and call. Hissing at the coil: call. Burning smell with any noise: shut it off at the breaker and call. Rattles, thumps and ticks: write down when they happen and call in the morning. We answer at 2 a.m. and the price is the same as 2 p.m.; the point of this page is that a noise sorted by timing is a noise we can fix on the first visit.