I am in the UK. I have an existing closed (pressurized) heating system with a boiler downstairs and the rest of the works + hot water tank in the cupboard upstairs.
Problem: when lots of radiators are open (they have electronic valves) + the hot water zone is on, the non-return valve emits a rather annoying "droning"/"buzzing" sound, but at other times it is perfectly silent. The valve was replaced recently on the assumption that it could be defunct, but the problem did not go away.
I suspect that air pushed from the radiators might get in the section of pipework highlighted in red and becomes trapped there, creating some sort of airlock, and leading to the issue I am experiencing. Is this a reasonable assumption or crazy talk? Would fitting an automatic air vent there help?