Related Clues:

• Sign of smooth operation
• Sound of a content cat
• Kitty croon
• Sound of cat (or engine)
• Run like a top
• Sing with lips closed
• Sound finely tuned
• Smooth engine sound
• Low murmur
• Result of some heavy petting?
• Sound from a contented kitty
• Play the kazoo
• Well-tuned engine sound
• Steady drone
• Beehive sound