I recently replaced one of my radiators and took the opportunity to
fit a TRV. (There are none fitted to any of the other radiators).

Now the radiator (or just the valve?) makes a loud noise when the
system is running - not quite a hissing noise - more like a "rushing
of water" type noise.

If I turn the TRV up or down a notch, the noise stops for 5 minutes.
then starts again.

Any idea what could be wrong?

Thanks,
Mark.