Cause of intermittent noise from pump and zone valves

[John.g]

Or maybe semi-sealed with a NRV on the cold feed, the vent blanked off and a E.vessel installed somewhere, very common around here as they are considered more forgiving than the fully sealed systems as long as the EV pre charge is kept to 0.5bar.

 

[SJB060685]

Yes it is a possibility and judge by the quality of pipework I imagine the system is a maze.