gearbox noise any idea?

Ok Chaps, I have just put the gearbox back in my series 1, i had it out to put new bearings in. when i drove it all was well for about a mile then there was a loud screeching noise, its speed dependent and will carry on if i coast out of gear. it comes on randomly and if i stop it will go away then start again once upto speed. if i run through the gears with the transfer box out of gear theres no noise. it sounds a bit like a metal shield on a bearing rubbing, but there are no shielded bearings. Any idea where to look first?

yes they are rear mainshaft bearing has oil thrower behind transfer box drive gear, and input shaft has oil shield touching bearing just behind lock nut ,it can be put on wrong way round sometimes