I can try this, but the pipes aren't hammering. We're getting a horn sound when the water is turned on. It does not happen all the time, but some of the time and it doesn't seem to be tied to any one faucet. It sounds like it is coming from the main water shut off line valve, the pressure reducer, or the cold water shot off valve going into the water heater. Would any of these valves be the source of the problem? Would air in the pipes cause this horn sound at these valvle locations?