Answer:

hums

AnswerCrossword Clue
HUMSRuns perfectly
HUMSPlays a kazoo
HUMSOperates perfectly
HUMSRuns smoothly
HUMSKeeps one's mouth shut?
HUMSSymptoms for a car mechanic
HUMSDrones
HUMSBustles
HUMSAccompanies musically sans words