Vailant customer service is abysmal, they clearly have a known issue with this unit (and probably the others in the range 828 and 835) since there are many folk on the internet with this problem AND the authorised engineers don't even bother to look for the problem when they arrive on site - they know EXACLTLY what to do to solve it.
I have made and uploaded a video recording of mine doing it.
Despite how obvious this is, if you experince the problem and email them they simply instruct that you call their 087 number. If you insist further on either more information or for them to quailfy the reasoning they claim the boiler is supposed to be very quiet and insist on sending an engineer. There is no mention or admission at any stage that this is a common problem.
I would have at very least have expected their staff to ask a few questions and establish that it was an expansion and contraction sound and then offer to send someone around explaning that it was a common problem. WITHOUT THE NEED FOR A PREMIUM RATE PHONE CALL. Even after referrring the matter to their Technical Manager (Tony) there is no admission.
So these folk are either incredibily dishonest or incredibly stupid. Either way VAILANT CUSTOMER SERVICE IS BAD.
I think this is the paste that was used (but I'm not 100% certain so if you're DIYing - at your own risk)
If your boiler ticks, call 017 7382 8100 say it's making a funny noise and when the tech arrives just tell them honestly what's happening (or refer them to this page).
They almost certainly have a little tube of copper paste and will dismantle the primary heat exchanger and re-assemble with the paste between everything.
This is yet another example of a really good product destroyed by it's after sales service.