An update.
At the last post, I was in Italy; I have now ridden up through Italy and France and made it to my brother’s place where I had access to a garage and a good selection of tools. So I took a few things apart.
Inspection of the speedo drive revealed that the circular gear would rotate freely in one direction, but not the other. Cleaning out all the grease freed this up and it would rotate both ways. I dismantled the gearbox and found that the worm gear had about 5-7mm free play axially; wear on the circular gear showed that this had been happening for quite a while. Inspection of the gearbox retainer showed that the dogs on the gear had been riding up out of their slots.
Because I still have to get to Belfast, I didn’t take the worm out to see what had happened there, but just replaced the housing without the innards in place.
I’d like to report that this has resolved the noise issue, but sadly, on my next trip, it was back with a vengeance.
It’s got me this far, so I’ll put up with it for a little while longer.