If you live in an hard water area the banging is almost certainly what is
known as "Kettling". This is caused by a build up of chalk/lime inside the
boiler. The build up reduces the water flow and volume with in the boiler
which will cause the water to boil thus producing the Kettling/banging. Some
time just reducing the water temperature or speeding up the pump will stop
the Kettling temporarily but if it is due to the furring up of the boiler
the only cure may be to replace the boiler.