I have a H100D propane pool heater that you guys helped get running last year and it worked great all summer ( minus a couple PH caused leaks). This year when I started it it works great but makes a banging sound only when the heater is on. When the no heat and just water flowing through it is silent. Other than the noise it works great. I took the heat exchanger out and cleaned what I could but there is a lot of green build up from the leaks. Any suggestions would be appreciated.