Related Clues:

• Good engine sounds
• Sounds of contentment
• Doesn't knock or ping
• Bustles
• Runs smoothly, as an engine
• Engine sounds
• Accompanies musically sans words
• Runs smoothly
• Contented sounds
• Forgets the words, maybe
• Satisfied sounds
• Hums like an engine
• Drones
• What a good engine does
• Is at a loss for words?