Glow worm 30ci loud clicking noise.

Hi
I have a Glow worm 30ci its about 3 1/2 years old but its always had a slight problem sometimes when the heating or the hot water is required and the boiler starts it makes a loud clicking noise (kinda like a machine gun). every time it does it I have to turn the boiler off and its fine after that but recently its happening more regular about 1 out of 10 times and it's not as loud.
The only thing I can think it must be is some sort of valve that switches between heating and dhw demand.

Does this make any sense to anyone? or has anybody come across this problem before I called Glow worm when I first noticed the problem as its in warranty they didn't have a clue and said if someone comes out and can't find a problem I would be charged.

Those boilers have a motor to drive the diverter valve and it may well be that.Fairly easy to change by removing circlip and unplugging motor.No drain down required although it is towards the back of the controls section and may be a bit awkward.

Thank you that sounds spot on and I will check that out.