"EricP" wrote :

As soon as my central heating pump kicks in - the very SECOND it does it -
then a very fast clicking noise can be heard in the bottom corner of one
of
my radiators - much before any HEAT is in the radiator so it isn't
expansion. It happens whenever the pump is on and stops when it goes off.

Is it a thermostatic valve on that end?

No it's on the opoposite end from the theramostatic valve.