Banging!

[Aithein]

Just had new combi boiler fitted (worcester) replacing the old back boiler. All fitted well and working fine, except now when the cold water is opened there is a loud bang, (only very slight bang on hot water) just the one then fine, no noise when closed . Can't locate the noise for sure but seems to be coming from the loft, checked new pipes, all seems ok .
The kitchen sink and washing machine water supply is only a foot or two of pipe to the mains supply downstairs so not sure why should that bang when opened?
Was all working quietly before so i'm a little confused as to what's causing it!
Any idea's before I have to call him back?
Thanks..

 

[AGAS]

try opening the stop tap fully

 

[Aithein]

Thanks for reply, just tried that, did open a little bit more but didn't make a difference to the bang!

 

[oldbuffer]

If its coming from the loft, then there is a possibility that the installers have connected the old storage tank inlet pipe to its outlet pipe as a way of converting to mains pressure cold water.

If the pipes are close together, or not well clipped, that could explain a movement when pressure is relieved by opening a cold tap. Solution if that's the case would be fairly obvious.