boiler making banging sound when main burner going on and of

[glittergirl]

I posted on here before and people were very helpful about my boiler and carbon monoxide. The carbon monoxide problem has been completely sorted now but I have a new problem. My boiler was fixed as it broke down completely and as well as fixing a leaking flue which was causing the carbon monoxide, the engineer had to replace something not sure what not that technically minded I'm afraid. Anyway all was well for about a week and although the boiler still works when the thermostat reaches the correct temperature and the boiler switches off the main burner it makes a banging/clanging sound not very loud but loud enough to be concerning. The same happens when the thermostat turns it back on again. It doesnt happen at all in the first hour or son after the boiler is switiched on (getting up to temperature I suppose but after that the thermostat turns the main burner on and off every 5 - 10 mins or so and the banging happens. It stops after a few seconds until it goes on or off again. Anyone any ideas what could be causing this? Many thanks.