update
gearbox has been stripped.
Grumbly input shaft bearing was found to be the noise I could hear.
The outer race face had failed on one of the diff bearings. This caused some filings in the box
fortunately this didn't cause any further issues and can simply be changed.
As long as the bearings and seals are reasonably easy to source, we should be good to go shortly.
I will let everyone know when I get a bit further forward.