Related Clues:

• Sound of silence?
• ''Zip it!''
• Librarian's reminder
• 'Don't let them hear us!'
• Volume control?
• 'Stop talking!'
• "Too noisy!"
• Audible silencer
• Peremptory signal
• Quiet "Quiet!"
• "That's enough from you!"
• Warning that often precedes 17-, 30-, 49- and 65-Across
• Gag order?
• 'Not a word!'
• 'No noise!'