The Annoy-a-tron generates a short (but very annoying, hence the name) beeps every few minutes. Your unsuspecting target will have a hard time ‘timing’ the location of the sound because the beeps will vary in intervals ranging from 2 to 8 minutes. The 2kHz sound is generically annoying enough, but if you really really want to aggravate somebody, select the 12 kHz sound. The battery life is 3-4 weeks muhahaha enjoy. I would love to see an open source hardware or wiring diagram for these things they don't look that hard to make yourself, if you have some spare time.

The Annoy-A-Tron is available to buy online for $9.99 (about £5) from ThinkGeek.
Direct Link here