My radiator pipes (old style/not baseboard) have been BANGING for over ten years. I mean loud, as if someone is hitting them with a baseball bat. This occurs even when the heater is not running...in the middle of the night when the thermostat is set low..when I burn a fire in the fireplace in the same room as the thermostat and heater is not running. It is a summer/winter system. The boiler unit was replaced 2-3 years ago and still have a problem. I have repeatedly bled the radiators and the expansion tank and never see any sign of air. None of the heater techs or plumbers I have asked can offer a solution...can someone help? It is driving me CRAZY!!