SEEMS LIKELY THAT THE DIVERTER VALVE IS PRODUCING THE "KNOCK"
THE VALVE WHEN NEW WILL OPERATE SMOOTHLY BUT WITH AGE AND VALVE BECOMING STICKY DUE TO CONTAMINANTS (LIME SCALE OR SLUDGE) THE RESULT CAN BE WHAT YOU ARE EXPERIENCING.
Thing is,that diverter valve was changed after just over a year.We noticed a rad in the kitchen was getting luke warm,without the heatng being on.Changing the valve sorted it out,cant be nothing else?.
I cant remember the system being drained.can it be done without draining,would it muck it up?there was no inhibitor put in i know that.there is now though.I'll turn the water down a touch,and see what happens.i'll check those pipes also.
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