Clunking Vaillant combi boiler

[diy wannabe]

My Vaillant combi boiler makes a bang of varying volumes (sometimes quiet, often like someone hitting the pipes once with a large hammer) when the central heating is on and I turn a hot tap on.

I understand there is some kind of shut-off valve for the CH when HW is required, but don't know much more than that. Should it be noisy, and if not, is it doing any damage and is it something I should get looked at?

Thanks, David

[htg engineer]

Definately the boiler ?

could it be the pipes banging - not clipped ?

[dogsbody]

My Vaillant Turbomax plus 824E does the same thing. The pipes hammer when the hot water is drawn or shut off as you open ot shut the tap only when the heating is on. This has only occured since the diverter valve was replaced. (faulty valve). Everything was OK at first then gradually the hammering became apparent and became noisier. The engineer was called back, he was puzzled, he drained the boiler fitted a new diverter valve and bingo all was ok. Gradually the fault came back, until now its as bad as ever. I need some information on this, first one up in the morning wakes the rest of the house with the hammer. It is obviously something to do with pressure in the system. The pressure was released by draining the fault went away until pressure builds back up again, I think.