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))