Loud clunking noise when boiler switches off?

I have recently moved to a new house. I have a Glowworm HX18 combi boiler. Each time the heating switches off via timer the boiler makes a loud clunk. The sound is amplified even more through the pipes which seen to rattle. I turned the boiler heat down to 70 degrees and the clunking stopped. This only lasted 3 days when the mains water to the house failed. Ever since the clunking noise and rattling pipes have returned. Lowering the heat of the boiler has made no difference. Any suggestions?