Sealed combi boiler with constant air
I have moved into our new home back in Jan and in Feb we had 3 of the 6 radiators replaced. Inhibitor was added and all seemed well. I've now had it since the system was installed that if, and only if, the study radiator upstairs (one which wasn't changed) is on, then the new radiator in our bedroom upstairs, and adjacent to the study, will garggle with air for about 3-4 seconds and then stop and the sound will not return unless the heating has been off for more than 3 hours or so. The sound is as if the air bubbles are trying to squeeze past the water in the column within the radiator at the flow in. This happens every morning when the heating turns on for the first time in the day. The pressure stays constant on the boiler and if I open the lockshield on the bedroom radiator more, then more air gushes in.
There are no obvious leaks anywhere on the ceiling below the study or bedroom, I have tried bleeding the air into a glass to see if it pops like hydrogen and it doesn't. The TRV and lockshield in the study both have green rust on the underside where the olive would be so water must have escaped at some point and I have taped it up with duct tape to no effect. I've also tried to nip the TRV and lockhield valves slightly to see if that makes a difference but nothing yet sadly.
Apart from going for the nuclear option of changing the valves and the study raidator (which I am considering to do in the next few weeks) - what could I do? What could it be? I've just increased the boiler pressure to about 1.8bar to see if that changes anything. The major puzzling bit for me is that it only happens when the study radiator is running and not when it's off on 0 on the TRV.