Solved (sort of)
Thank you Paul and Box for your assistance.
I think I've finally tracked down the problem.
The two offending notes were A5 and Bflat5 corresponding to 1st and 2nd row buttons (CBA).
I removed the A pallet which was very easy, just slid it off the rubber coupling and wiggle it out.
The pallets are in perfect as new condition so that was not the cause.
I did find however that the clack was still there with the pallet removed so the problem was in the linkeage.
I took the underside panel off the keyboard to expose the linleage and noticed that on all the repeater buttons i.e. rows 4-6. the pistons were very close to the upper edge of the hole and in the case of the A and Bflat buttons, actually made contact with the base plate when released quickly, although it didn't happen on "normal" playing, only when playing triplets etc.
With carefull use of Swiss files I was able to open up the hole that the piston went through on the A button and the clack disappeared- so much so that all the other buttons sounded noisy!!

. This must have been a fault from new which wasn't picked up as it only happened playing triplets .
I was unable to completely fix the Bflat note as I was unable to reach the piston/hole to file it. However, as I'm not likely to do too many triplets on the 2nd row and it wasn't as bad as the A, I'll leave it.
Also, all the other notes which now sound noisy compared to the one I fixed will be left alone.
Well spotted Box, on the musette couplings.
I originally bought this box as it was advertised as LMMMH as per the couplings, but, on arrival and inspection, it is LMMH, the MMM musette button is in fact MMH.
I didn't return it as it is a very nice accordion, and, apart from a few minor problems, virtually as new.
It was bought new by a man who had played piano accordion all his life and, on retiring, decided he would like to try a CBA.
After a couple of weeks he found he was too set in his ways with the piano accordion and this one was consigned to a cupboard where it remained until he passed away.
His wife needed to dispose of his accordions and his friend, also and accordionist (piano) since childhood advertised them on her behalf, which was how I came to buy it, as I was looking for a decent LMMM box and this seemed to fit perfectly.
I'm very happy with the box, which, apart from a few minor problems not helped by being stored away (luckily dry) in a cupboard for about 10 years, such as a few valves needing re glueing etc. were all easily fixed and I have an as new accordion for a very reasonable price.
However, I've now got my eye on a lovely Borsini CBA, LMMM, with midi and mikes, newly refurbished, so things may change

I've enclosed a couple of photos to show what I've been blethering about.
Sorry 2 had to be as attachements as it said wrong format.

thanks again, Ian
