An Odd Instrument: Saltwater and Aluminum

nitro2k01 from Gameboy Genius has a theory:

The cupper and aluminum works as a battery, much like the lemon battery taught in physics class. However because of the Al2O3 surface on the aluminum, which is released in an opscillating manner, you get spikes of voltage. This is what creates the sound. Compare it to the bubbles you get when you put a piece of a base metal in acid.

Source: Peculiar Sounds from Aluminum