i have a combi boiler in the bedroom, the pipes that go from the boiler down into the living room are rumbling quite loudly. I dont hear it in the bedroom, just in the living room where the pipes come down in the wall. It is not a constant rumble, it comes and goes throughout the day and night. It seems to be when the heating is switched on, even if the thermostat hasnt actually clicked the heating on (i.e. rads may still be cold). In the summer months when the heating is not used i get no rumbling.
I have bled the rads, although one rad was inaccesible, im unsure if that caused the bleeding to be a useless task. Any ideas??
I had the same issue at my fathers house with his heating system. His thermal expansion tank had failed. I replaced the expansion tank, bled the heating system and problem solved. This may or may not be your case.