Related Clues:

• Cat sounds
• Signs of kitty's contentment
• Acts contentedly
• Sounds from a Siamese
• Sounds contented
• Doesn't knock?
• Smooth engine sounds
• Abyssinian's sounds
• Betrays contentment
• Runs well
• Doesn't ping
• Contented comments from 27 Down
• Sounds content
• Hums
• Well-tuned engine sounds