One of our pipes underneath the floorboards used to make a noise due to expansion when the heating was on which affected one of the clips which was holding it in place; the pipe vibrated and hit the clip and the floorboard above which made a banging noise. I dont know if this helps but it may some thing like that thats wrong rather than something inherently wrong with your system, however if there is air in your system it may need a full drain down as bleeding radiators only really resolves the problem temporarily in a few locations. Air can get in to the system at any time through faulty TRVs ie thermostatic radiator valves and given the considerable length of pipework in any home getting rid of air blocks is not straightforward. The main indication of air is when radiators are not heating through properly, ie cold at the top/bottom , hot elsewhere, not just noise. However the best advice is to contact your plumber as they really should be sorting this out for you.