The back boiler on my coal fire keeps banging when the water heats up.?

I have a back boiler on my coal fire. This is wired in to my control panel so that when the back boiler heats up I can press the override on which cuts out the LPG heating. When the water in the back boiler heats it I can hear it bubbling and then the back boiler keeps making banging noises. After reading reports on various websites I am trying to establish whether this banging noise is correct.