J.—-It is not unusual for gas fired water heaters to make strange noises. Sometimes sounding like huge concrete eating termites gnawing away at the foundations of our homes. The crunching noise you hear is due to steam bubbles generated at the bottom of the heater when the burner is on. As soon as the bubbles start to rise, they encounter cooler water. The change in temperature causes the bubble of steam to implode back into water in its aqueous state. This is in part caused by sediment that accumulates on the bottom of the heater and acts as a thermal insulator, and so the water is is flashed into steam. A thorough cleaning of the heater will help control the noise……..or then again, it may be Giant concrete munching termites..Merry Christmas… Bud.. Suncoast Plumbing