Banging is usually due to an air-lock in the central heating system, if you follow the commisioning sequence as outlined in your boiler manual you should be able to remove the air in the system. typically, all you need to do is switch off and open the unit there will be an air bleed screw located near the pump turn it remove til lwater emerges.then remove front cover plug from the pump and rotate the pump shaft by 180 degrees using a flat head screwdriver,till water is seen smoothly coming from the system.then retighten the screws and refit the front cover.
Dave Penson
March 2006