Answer:

hum

Answer	Crossword Clue
HUM	Drone
HUM	Ho follower
HUM	Motor's sound

HUM	Running sound
HUM	Buzz
HUM	Run smoothly
HUM	Unwanted speaker sound
HUM	Droning sound
HUM	Speaker interference
HUM	Bustle
HUM	Karaoke singer's filler

HUM	Sound system problem
HUM	Substitute for forgotten words
HUM	Hive sound
HUM	Sing, in a way
HUM	Sing with one's mouth shut
HUM	Fridge sound
HUM	Sign of smooth operation
HUM	Whir

HUM	Be at a loss for words?
HUM	Sound system noise
HUM	Sing without words
HUM	Busy state of activity
HUM	Drum intro?
HUM	Purr
HUM	"Rattle and ..." (U2 album)
HUM	Smooth engine sound

HUM	Run smoothly, as a machine
HUM	Carry a tune without words
HUM	Low monotonous noise
HUM	Play the kazoo
HUM	Sing sans words
HUM	Busy sound
HUM	Forget the words, perhaps
HUM	Run well

HUM	White noise, perhaps
HUM	"Rattle and ..": U2 album
HUM	Engine room noise
HUM	Be busy
HUM	Forgo the lyrics