The quiet whirring of a fan in the background is an example of a hum.
The small, steady noise your speakers make when your amp is acting up is an example of a hum.
The avenue hummed with traffic.
A room humming with voices.
To hum a child to sleep.
They could hear a hum coming from the kitchen, and found the dishwasher on.
We are humming happily along with the music.
To hum a tune.
The hazers ominously hummed "We shall overcome" while they paddled the unruly pledges.
When a fan whirs quietly in the background, this is an example of when the fan hums.
When a cafeteria has a low, steady sound of chatter, this is an example of when the cafeteria hums.
When you make "hmmm.... hmmmm... hmmmm" sounds to the beat of a song, singing without opening your lips, this is an example of when you hum.