Related Clues:

• Bustles
• Runs smoothly, as an engine
• Engine sounds
• Accompanies musically sans words
• Runs smoothly
• Forgets the words, maybe
• Drones
• Is at a loss for words?
• Runs well, as an engine
• Doesn't know the words?
• Buzzes along
• Sings with closed mouth
• Close-mouthed singing
• Sings along without words
• Is abuzz