I have one that rattles along with the pump frequency. I have swapped it
from end to end - can't remember if I have done every permutation of
turning it through 90degrees - currently it is horizontal.

It seems the microbore pipework is very susceptable to any shock on the leg
of the radiator. Pressing the pin manually will cause a distinct shockwave.

Wondering about trying to tighten the gland nut - or removing it and
packing some grease in.

Any ideas? I guess clipping the pipe bettermight help - but I ain't lifting
a chipboard floor.

The locksheild is turned right down as well.