Flourescent light switched on with a flicker, transformer hum, light switched off
Inside a kitchen a fridge door closes with hum of fridge in background.
Big desk fan, low, soft, hum.
Laser printer warming up beginning with flicking the on switch button on and hearing the motor and fan ...
Buzz type motor noise that oscillates and fluctuates with tiny clicks in it
Pulsing hum with flutter, pitched down at end - motor off, dying engine.
Humming sound of soda coolers running in the store
Hum of an extractor fan with background ambiance