flushing it out and adding new gear oil would be a good start, might help quiet it down, but it may need an overhaul, the bearings could be shot, and if it's grinding into 2nd and/or 3rd, the syncros could also be a problem .... if you do need to swap, you could always use the gears from the noisy one in the other, maybe make one good out of the 2
Bill
