Related Clues:

• Sound of unvarying pitch
• Pong
• Sound (of business?)
• Single unvaried sound level
• Drone — stink
• Fail to know the words, maybe
• Sign of smooth operation
• Sing with lips closed
• Smooth engine sound
• Low murmur
• Play the kazoo
• Steady drone
• Beehive sound
• Unvarying pitch
• Sound of busy activity