faint, gentle, soft
low
high-pitched
constant, steady
I could hear the constant hum of distant traffic.

background

hear

hum of
the background hum of the air-conditioning

quietly, softly
loudly
happily
tunelessly
along
I was humming along with the music.

begin to
He began to hum, somewhat tunelessly.

hum to yourself
She was humming softly to herself.

Hum is used with these nouns as the subject:
bee, economy, engine, machine, refrigerator

Hum is used with these nouns as the object:
bar, melody, snatch, tune