Short loud rapid hammering noise when heating pump starts

[Cheesebo]

Hi,
I have a Vaillant ecoTECT plus combi boiler (I think it's a 618). It was installed around 7 years ago and has been serviced on a regular basis. Up until a month ago (a month after it's last service) it has been working perfectly, but now it has started to make a very loud, rapid hammering noise for about a second as the heating pump kicks in (when the heating is switched on, or when the heating is on but the hot water tap has just been switched off).
The noise only happened intermittently at first, but now it happens nearly every time.

Could there be a simple fix for this, or do I need to get the service engineer back in?

Cheers,
Dave

 

[Ermintrude]

Just take a few trv heads off? Possibly? What's pressure in boiler?

 

[gasmanrob]

Possibly knocking of the diverter valve when is actuated over the pump could be slamming it in place

 

[Ermintrude]

It's got a built in bypass I was thinking it was overrun and trvs . Bypass set too high?

 

[gasmanrob]

But that would be more then a short knocking I think and for trv's to chatter bypass would surly need to be close to closed or blocked maybe?