If my boiler is turned on having been turned off for a while, it fires up happily. After 30-60 secs it switches itself off. The outbound pipes then proceed to make the most horrendous banging noises. The outbound pipe is split in two - one route goes to the attic, and is very hot, the other I presume goes under the floor boards and is cold. The pipes to the attic are the ones making all the noise. Anybody got any ideas?