Noisy Vokera Excell 80SP!

I have an 11 year old Vokera Excell 80SP which I inherited when I moved into my flat in the south side of Glasgow this year (service history unknown).
My problem is that when I initially make any demand for CH or DHW from the boiler when it has been switched off, there are some swooshing and mechanical clunking noises. Once it is up and running it is fine in either mode. Would I be right in thinking the pump or perhaps the diverter valve may be to blame.
The fault isn't urgent, as with the winter coming upon us the boiler will most likely be left on almost constantly.However if it is a sign of something about to fail then it would be better correcting the fault now.
Any advice would be gratefully accepted.

glasgow vokera DP will have a field day i would hang on fella

Thanks for a few kind words Kevin!!

Boiler should NOT be switched off i.e. the left hand control MUST not be in off position if the pilot on.

Boiler will work better with a service. Rare to find a boiler that has been attended to as specified by the makers. 15 minutes look see is not a service

Thanks for the advice, especially the part about it the mode selector not being in the off position when pilot is on.
I will need to save my pennies for a proper service by the sounds of it.