Related Clues:

• Sounds of contentment
• Doesn't knock or ping
• Runs smoothly, as an engine
• Engine sounds
• Runs smoothly
• Contented sounds
• Satisfied sounds
• Hums like an engine
• What a good engine does
• Engine hums
• Expresses pleasure
• Murmurs contentedly
• Contented cat sounds
• Runs sweetly
• Cat sounds