Boiler noise, Serious?

[Road2Ruin]

I am usually quite good with plumbing but this has got me stumped. We have a Vaillant combi boiler that is about 5 years old. When we turn the hot water tap on, in any room, there is a loud clunk from the boiler that reverberates around the plumbing, making it sound 10 times worse. This only happens when the heating is also on. Could it be a sticking 3 way valve or something more serious? The whole system still works but I dont want to leave it until its too late.

Cheers
Pete