Do you get disturbed by unnatural sound? Take a look, there might be annoying buzzer hidden near by. If you are lucky not to have one, then be nasty and do this crazy prank to your friends. Chris have put a small buzzer which generates short sound bursts every few minutes.
The pulses are short and scattered in time so it would be hard to track the source. Its small size allows to dig it deep in room mess. Buzzer is based on PIC10F202 microcontroller, coin cell battery, switch and speaker from small headphone. The PCB is practically same size as battery. No matter how tempting is to place this thing everywhere around, be responsible and creative.