A friend of mine has said that they frequently hear "booming" noises coming from their water heater. They have adjusted the temperature up and down, and drained and refilled the heater, and it still occurs. It is just an occasional thing. The noise sounds like a big, low, boom, almost like a very large air bubble popping deep under water. Anyone have any ideas what might be causing this and if there is a fix (short of replacing the heater)?