Rheem Solar Loline (gas boost) 511270-G

My Rheem Solar Loline in our 3-year-old newly purchased home made itself known by refusing to supply warm/hot water. A service Tech kindly explained that blocked mixer and tap filters were keeping the unit from boosting as a certain number of litres per minute were required to kick it off. Removal of tap filters remedied the problem. We however have a big problem with the groaning of the pump unit as it cycles water at seemingly random times. The groaning is some times cyclic and sometimes sounds like a speaker buzz. Why it operates at 11pm is a mystery to me, fine when it moves cool water up to the panels to be warmed but one would only expect that in sunshine hours.
The groaning noise is further disconcerting as it emanates from the wall behind the entertainment unit where the pipe traverses from the tank up into the roof. It strikes me that these things need better sound insulation on the pipes.