Related Clues:

• Performs musically sans words
• Runs well, as an engine
• Doesn't know the words?
• Buzzes along
• Engine hums
• Expresses pleasure
• Murmurs contentedly
• Contented cat sounds
• Runs sweetly
• Cat sounds
• Sings with closed mouth
• Close-mouthed singing
• Signs of kitty's contentment
• Sings along without words
• Is abuzz