A simple noisemaker circuit, originally published in a Radio Shack DIY book in the the 1970s. It consists of a square wave oscillator, which acts as a trigger for a pulse generator, which produces the audio output. The simplest version of the circuit has two controls, one for the frequency of the oscillator, and one for the pulse width of the pulse generator. The circuit is based on a pair of 555s, a basic and inexpensive timer/oscillator integrated circuit, and can be built with about $30's worth of parts. Kits are available from a number of vendors.
A large number of modifications and additions exist and can be found in various places on the Internet. The circuit is popular with experimental groups performing live, and within the "maker" community. The circuit can be made on a small circuit board, and often is built using unusual objects as enclosures; it can be powered using a single 9-volt battery. Below is a video of an APC built into an Altoids box: