Related Clues

• Accompanies musically sans words
• Bustles
• Runs smoothly
• Drones
• Symptoms for a car mechanic
• Indistinct sounds of mingled voices
• Purrs
• Is busy
• Is abuzz
• Sings along without words
• Sings without words
• Is at a loss for words?
• Joins in wordlessly
• Runs well, as an engine
• Sings close-mouthed
• Sings with a closed mouth
• Operates smoothly
• Doesn't quite sing
• Emulates a bumble bee
• Obviously doesn't know the words