Every time I flush a particular toilet , after the chamber refills and the float valve stops, there is a knocking in the wall behind the toilet. Other toilet and valves do not cause knocking. I have an old valve (40yrs) in place ; not a ball valve on a lever, but a vertical valve with a plastic red top. Whenever I push down slightly on the shut-off valve, the knocking occurs. Can this be fixed by replacing the valve or should I replace the entire flush mechanism?

I would replace the valve first, Good luck, Tom