Possible Clues:

• Sing along without words
• Keep one's mouth shut?
• Make music on a kazoo, say
• Noise from a beehive
• Sound of activity in university Her Majesty is going round
• A sign of business that is open to question
• What many air conditioners do
• What to do when you don't know the words to a song
• Wordlessly sing
• Do less then sing