Possible Clues:

• Noisy crime prevention device
• Security measure
• Sound system popular with The Police has a setting for The Doors presumably
• A loud type doing security for The Doors is primed to put gatecrashers off
• Noisy warning (2 words)
• By the sound of it, one primed to put gatecrashers off is a loud type doing security for The Doors
• One there for the taking raises red flag? You often hear that in relation to unwelcome visitors!
• People regarded as thick may ring this bell