the sound is created by a combination of gas pressure, the fact that it's actually burning, and the design of the muzzle. An oxygen/acetylene cutting torch makes the same sound, especially when you kick in the main cutting jet.
I'm sure that you could get a gas mix that would work without having to use cc rocks and water. Any welding supply dealer can mix gases.
I would even go as far to run a remote from a 12oz or 16oz bottle to feed the dsfg. Probably get 30-40 shots out of it. Just hide the tank connect in the gas chamber. Remove to cap to fire repeatedly.
P.s. Got my sandman shirt ready for tonight's party.. Two t-shirts, some seam glue, and walla..