Need help with mystery noise

[asylumdown]

Mystery solved!

During the day, my turbo snails cluster against the glass beside the heater in the corner where it's shaded. I was finally looking right at one when it puffed it's shell far away from it's body and started spinning it rapidly. Now that I know to watch, I've seen them do it a couple of times. When the shell only knocks against other snails (Is it a territorial thing?), it makes a softer, more muted noise. When it's right against the glass, the knocking sounds like the tank is about to bust open.

It sounds like the sounds is coming from 2 feet from where the snails are making the noise. Acoustics are nuts.