After the central heating has been turned on and the boiler has ignited there is a whooshing sound followed by constant banging and clanking coming from pipes close to the boiler. There are also noises which sound similar to switches or valves turning on and off. I immediately turn the heating off as it sounds as if it is about to bring the house down, and the banging stops after about 20 seconds or so. Does anybody know what is going on?