Meaning of “hum” - Learner’s Dictionary

hum

verb uk us /hʌm/ (present participle humming, past tense and past participle hummed)
SING [ I, T ]

to sing without opening your mouth:

She hummed to herself as she walked to school.
I've forgotten the tune. Can you hum it to me?
MAKE SOUND [ I ]

to make a continuous, low sound:

The computers were humming in the background.
be humming

If a place is humming, it is busy and full of activity.

hum noun [ C ]

a low, continuous sound:

the hum of traffic

(Definition of “hum” from the Cambridge Learner’s Dictionary © Cambridge University Press)