Selgas
The Rinnai water heater you have installed (you call them tankless units) is a mains pressure device that has no pressure reducing valves fitted in the line to it and is unlikely to be the cause of your problems.
The banging noises you are hearing are more likely to be those of fast opening or closing solenoid valves or loose fitting pipework.
Household items such as dishwashers, washing machines, wastemasters and the like often cause this water hammering by their electric solenoids slamming shut – effectively causing a water shock wave within the pipework.
Check if there are any of these type of devices fitted to any of your pipeline and then check if they are operating when such noises are heard.
