Possible Clues:

• They're not to be seen if they're off
• On the face of it, they need one inside to make them as sound as they are
• Bangs, clacks, e.g.
• Pops, e.g.
• Murmurings
• Spreads abroad with sounds
• See 20 down, by the sound of them
• To such one 29 across
• Boom and zoom
• One hears that they get a smell about one