Does the rattle occur when driving straight or when turning? My differential clunks when I do sharp turns on 1st or 2nd gear. In my case, it only happens at slow speeds.
At first I thought it could be the tyres, but the problem still happens after replacing all four identical tyres. Mine is a 3.0 TD. I will be replacing the differential oil in the next few week and will let you know if that fixed my problem.