I dont know the exact reason why, but all gas water heaters make that type of noise when they get a build up of scale in the bottom of the tank. The water is pretty hard here in San Antonio and it only takes about a year for a heater to start making that noise. Flushing the tank might help, but I wouldnt bet on it. Most people just live with the noise, it doesnt seem to damage the tank at all.Maybe someone else will give more info.
