This only happens when the boiler starts up from cold. The noise will recur when the system is left to go cold again ie. if all the rads are turned off, when a rad is turned on it triggers the 'cold starting' noise again. When running normally there is no noise apart from the very quiet normal running noises. This noise also occurs when running hot water from a cold boiler.

I think it is caused by the expansion vessel but I can't be sure. any suggestions ?

It may be the expansion tank, you need to contact the manufacturer.

Should my gas central heating boiler makes a lot of water noise in operation?