Real Dictionary

hum
verb: If something hums, it makes a low continuous noise.

hum in American English
interjection: (an inarticulate sound uttered in contemplation, hesitation, dissatisfaction, doubt, etc.)
intransitive verb: to make a low, continuous, droning sound
noun: the act or sound of humming; an inarticulate or indistinct murmur; hem
transitive verb: to sound, sing, or utter by humming

hum in American English 1
to make a low, continuous, murmuring sound like that of a bee or a motor

hum in American English 2
hem2

hum in British English
verb: to make a low continuous vibrating sound like that of a prolonged m