Some time after a radiator TRV is switched to cold, it begins to whine. This is very loud. When I turn the radiator on, a bubbling noise is heard and the radiator stops whining. Radiator is bled, and there is quite a lot of air - probably from the bubbles. Radiator is again switched to cold when not required. Cycle repeats. What gives?
I'm a bit worried about draining all the pressure from my CH system if I have to bleed the radiators a lot.