gearbox

I own a 1984 300GD with a 5 speed manual gearbox (717).
If the gearbox is in neutral and I release the clutch, a loud rattling sound emanates from the bell housing/gearbox area. Originally, I thought it might have been a problem with something in the clutch but after having worked out how a clutch works, I'm now thinking it could be something in the front of the gearbox; possibly something to do with the splined shaft.
Is anyone familiar with these vehicles? Can anyone suggest what my problem may be?
Is this gearbox specific to the G-Wagon only or is it the same box in some of the mercedes cars?
Are parts expensive?(rhetorical question - of course they are).

Any help is greatly appreciated.

Oli

P.S. I'm in Australia so parts can be extremely difficult to come by.