Hi Harold didn’t I ask the same question RE banging?
Of course there is always the very remote possibility of a Quick closing valve causing hydraulic shock waves like using a hydronic’s system where the pneumatic controllers close in the fail safe mode Or someone using a gate rather then a globe pattern electronically controlled zone valve.
Then there is always the possibility the system is indeed a non vapor system and the flow control valve is spring loaded and when the circulator shuts off it closes too quickly.
Of course there is always the problem with excessive velocity combined with higher temperatures and the mixing of dissimilar piping materials and the coefficient of expansion per degree per foot can more or less a pinging sound.
How about a a PRV failing or the relief valve going off on excessice pressure and may be closing way to fast?
Could it be a 9D back flow preventer is near a swing check rather then a lift or ball or soft seat check valve?
How about Harold we keep on guessing what could be wrong UNTIL we actually find out that this is REALLY a vapor system and someone gave us erroneous information HUH?