OK - - It's fixed. As I suspected, the MANIFOLD VALVE was at fault. The "piston" in the valve had broken off and left a permanent opening for cold, inlet water to bypass the heating coils and go directly to the outlet pipe. This didn't leave a sufficient amount of water in the heating coils to avoid steam from forming - - which is the banging noise I was hearing. Additionally, the broken piece had been flushed out of the unit; downstream, towards the pool where it snagged at the next (Jandy style) valve. This was causing a (back) pressure build up at the pump/ filter. Amazingly, there was still plenty of flow to the pool. I have attached a couple of pictures. The first is a shot of the loose piston, next to the jandy valve which was opened to gain access to the pipe. The next is a photo of the new manifold kit on the left. Next to the damaged piece, missing the piston.