Hell Is That Noise/Theatre

The shrill, piercing factory whistle in Stephen Sondheim's Sweeney Todd: The Demon Barber of Fleet Street is first used to make the audience jump and shut up in time for the opening number. Every subsequent use of the thing gets more and more hardcore - Sweeney's first kill, for instance - until the final use of the factory whistle coincides with Toby killing the main character in the world's creepiest Freak-Out. Yeah, you're still going to jump when you hear it on the soundtrack later.
- And for those who haven't heard it, it's really shrill and piercing. If you're listening to it on your iPod with earbuds in, the sheer high-pitchedness of it will make you jump out of your skin no matter how many times you've heard it and how much you're expecting it.