'88 930

I have noticed a worryingly expensive sounding noise eminating from (I think) the gearbox. It ONLY happens at 1100 - 1300 RPM and is not related to road speed. Also, it is only apparent ingears 2 and 3. It is a kind of rumbling graunch. Someone suggested the layshaft case hardening has worn down. if this is so, is it really a big job to whip out the 'box and pull it to bits ?

Thanks for any help.