Loud Rattle on Startup That Fades: Motor Bearings Worn Out

A harsh rattling from an indoor unit at every fan setting, rising in pitch as the speed increases. Nothing on the casing or the trunking stops it when held firmly. Cooling is entirely unaffected.

Running the fan through all its speeds separates the two families of cause without any dismantling. Something loose usually has a threshold: it is silent below a certain speed and appears above it, or it fades once the airflow settles. Something rotating with excess clearance makes noise at every speed, and the pitch tracks the speed continuously because the source is the rotation itself. That second pattern points at the motor.

A fan motor's bearings hold the rotor in a precise position, and that position sets the gap the rotor turns within. Wear opens that gap up, so the rotor is no longer held centrally and touches or drags as it spins. The sound is produced by the rotation itself, which is why it never disappears and why its pitch follows the speed so closely. Nothing about it improves with cleaning or with tightening.

The motor is replaced, which is a genuine part and a genuine cost, and it is the right answer once the cheap candidates have been eliminated in the open. Working through the fan speeds first is what makes that credible. A motor quoted without those checks is a guess dressed up as a diagnosis; one quoted after them is a conclusion anybody can follow for themselves.