Knocking pipes / Rads

Hi
When my system is operating for 2-3 hours and the Thermostatic rad valve in any room is half shut we get a knocking in the system. Going to the half open valve and either fully opening or shutting it down stops the knocking.
This means we have to have almost all the rooms fully heated or cold.
The valves are not sticking shut as we get plenty of heat.
Any ideas please?
Richard

They are probably fitted the wrong way round.

There is a directional arrow on each valve. The heat should travel in the direction of the arrow.

Modern TRVs are Bi-Directional with an arrow that points both ways and it doesnt matter which way round they are.