Hi you have the correct code but I don't think its the box, it sounds as if it might well be a clutch problem, thrust bearing, flywheel, first motion shaft bearing, drive shafts won't stop rattling if you push the clutch down, especially when you are not moving.
Steve.
It would be a mistake, and an expensive mistake at that, to change the gearbox until it has been confirmed as the source of the knock. The noise could equally well come from the clutch or the drive shafts (CV joints), neither of which will be improved by changing the gearbox.