Answer: HUM

AnswerCrossword Clue
HUMSing sans words
HUMSing without words
HUMBusy state of activity
HUMDrum intro?
HUMPurr
HUM"Rattle and ..." (U2 album)
HUMSmooth engine sound
HUMRun smoothly, as a machine
HUMCarry a tune without words
HUMLow monotonous noise
HUMPlay the kazoo
HUMSound system noise
HUMBusy sound
HUMForget the words, perhaps
HUMRun well
HUMWhite noise, perhaps
HUM"Rattle and ..": U2 album
HUMEngine room noise
HUMBe busy
HUMForgo the lyrics
humto make a sound like bees
HUMKaraoke singer's filler
HUMHo follower
HUMMotor's sound
HUMRunning sound
HUMBuzz
HUMRun smoothly
HUMUnwanted speaker sound
HUMDroning sound
HUMSpeaker interference
HUMBustle
HUMDrone
HUMSound system problem
HUMSubstitute for forgotten words
HUMHive sound
HUMSing, in a way
HUMSing with one's mouth shut
HUMFridge sound
HUMSign of smooth operation
HUMWhir