I noticed our vent hood exhaust fan has become unusually loud and most noticeable at low speed. When it starts up, there is an audible hum and periodic ticking/vibration that increases in frequency and can still be detected at low speed. It you put your hand on the motor casing, you can definitely feel it.

Strangely, when you power the fan off and let it spin down, everything is smooth and quiet. It only makes the vibration when power is applied.

Any ideas? It's a relatively simple system - transformer, capacitor, and motor with a switch.

Thanks for any help.

Jimmy