I had a somewhat similar (but not as bad) sound when my trans died last year. Exactly as warp mentions, circlip on 3rd gear migrated out of its' groove leading to catastrophe.
When splitting the cases you'll need the special tool to remove the flywheel (over the stator) at least, the rest can be lived without. Don't try to split the cases with the stator in place, there's a hidden bolt behind it that holds the halves together...