I'd check the prop and driveshaft uj's, and the diff pinion mount and bearer plate, because it's always worth doing that, but if it's definitely from the gearbox and third gear only then it could be something in the third/fourth synchro hub broken up and fouling, or if the noise is there in first and second as well but less so then it could be the rear mainshaft bearing. The hubs do suffer with bits chipping off, and the cages can break on the bearings allowing the balls to move closer together, forcing the cage outwards and making that catch somewhere. Either way, if it is internal, with the propensity of those boxes to grenade themselves, taking it out for a look sooner rather than later is a good idea. If it is internal, with a noise that loud the cause should be obvious when it's in bits in front of you.