Possible Clues:

• Happy cats do it
• A stroke of the cat may produce it
• Contented sound of a cat
• Copy cats?
• Kitten's call
• '17 across, perhaps, is happy, by the sound of it (4)'
• Sound of a contented feline
• Sound of a fine-tuned engine
• Sound of happy cat
• Engine sound backed up Rolls-Royce