Related Clues:

• Cat or engine sounds
• Purrs like an engine
• Sings without lyrics
• Hums like a tuned engine
• Good vibrations?
• Makes like a happy tabby
• Purrs, as an engine
• Audible responses to catnip
• Sings without singing
• Goes "mmm mmm mmm mmm," maybe
• Plays the kazoo, say
• Beehive sounds
• Sings to oneself