Related Clues:

• Warning that often precedes 17-, 30-, 49- and 65-Across
• Gag order?
• 'Not a word!'
• 'No noise!'
• Theater sound
• Theater reproof
• "Quiet down!"
• Silencer's sound
• Librarian's rebuke
• A sound for silence
• 'Stop chewing on that goddamn apple
• Finger-to-lips sound
• 'Put a cork in it!'
• 'I'm trying to watch 'The Force Awakens' here, shut up!'
• 'Ahem' subtext, maybe