I sometimes hear a noise that sounds like bubbles entering my bathroom
radiator. Thinking there must be an airlock moving I opened the lockshield
fully but the noise stopped.
I opened the air vent but no air came out.

This has happened several times - there is never any air to bleed out. I
mainly hear the noise after a shower when the combi's motorisedvalve would
be going back to heating mode.

Any ideas? (the rads are fed by double enty valves (Maxtwin))