Related Clues:

• Is abuzz
• Symptoms for a car mechanic
• Sings without words
• Ambient sounds
• Joins in wordlessly
• Sings close-mouthed
• Runs well
• Sings with a closed mouth
• Operates smoothly
• Hi-fi annoyances
• Sings with closed lips
• Emulates a bumble bee
• Obviously doesn't know the words
• Is busy
• Keeps one's mouth shut?