"humming" in the noun sense
1. hum, humming
a humming noise
"the hum of distant traffic"
2. humming
the act of singing with closed lips
"humming" in the verb sense
1. hum
sing with closed lips
"She hummed a melody"
2. hum, buzz, seethe
be noisy with activity
"This office is buzzing with activity"
3. hum, thrum
sound with a monotonous hum
4. hum
make a low continuous sound
"The refrigerator is humming"
Source: WordNet® (An amazing lexical database of English)
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