I am a builder, my neighbour has a combi boiler fitted in the loft with the flue through the tiled roof. i noticed that it was making a bang from the flue pipe. i did tell the neighbour that i did not know enough to say what it was but i knew that it wasn't right. they don't seem to have done anything about it and it seems to be getting louder and seems to be when ever the boiler fires up. anybody got any thoughts on this