Related Clues:

• Sing sans melody
• Plane sans pilot
• Go on (and on, and on ...)
• Queen's mate
• Unchanging intonation
• Ambient sound
• Long hum
• Talk in a monotonous voice
• Go on monotonously
• Remote-controlled plane
• Mindless worker
• Monotone
• Ongoing monotone
• Many a bee
• Play monotonously