By the sounds of it its most likely to be the gearbox bearings but i can't be 100% sure without hearing it. BUT it could also be a thrust bearing or spigot bearing and if i was doing the job it would be a strip and quote because you won't know if its due to a clutch fault or internal gearbox fault.

If you can't do any of the work yourself i would be prepared for bill of up to 5g's as an extreme worst case!!