Rattle! Help required?

I have a GAs Combination Central Heating system. The Radiator Valve in one of the rooms rattles now and then. Whilst nothing has happened over the years ( movement etc) occasionally it "vibrates" when the heating is on. This vibration goes right through the system. If I press the top of the valve, the vibration stops, If Iturn it down it stops, although not for ever

Can anyone identify the problem. Would I have to drain the system and replace the valve? The valve is vertical and located at bottom right of radiator