I have a condo with a 10 year old, 40 gallon electric water heater. I have city-supplied water. Obviously I'm due for a new water heater real soon. I live in a region with very hard tap water. Whenever a water valve is open in the condo (toilet, sink, or washing machine) there is a repetitive banging sound coming from the walls. I can also hear the sound in the water heater. The repetition rate is roughly 2-3 Hz. Some days the sound is so loud you'd think my neighbor is pounding on the walls with a hammer. Someone at the local water company suggested to me that the problem is due to an air bubble in the water heater arising from calcification of the heating element. That sounds plausible to me, but how does an air bubble in the water heater cause pipes to bang? What is the device in my house that is opening and shutting repeatedly and causing the banging sound?