Everytime the new power vented gas water heater cycles on, there is a knocking/clicking sound that comes up every four seconds or so which appears to be coming from the 3" PVC exhaust pipe from the water heater to the outside. I am guessing the hot air is expanding the pipe, but I don't know what to do about it. In fact, I don't even know who to call to make the repair. A plumber, HVAC guy, General contractor or someone else. Can somebody help me out?