Alright, the engine mounts sagging have nothing to do with gearbox bearing wear. From what youve just said it does sound like a input shaft bearing especially if it doesnt make noise in gear foot on clutch ( input shaft not spinning). I have done gearbox bearings before but highly recomend not pulling it apart unless your familiar with how they work. Before you jump into anything as it will be extremely costly to take it somewhere to have done, try changing the oil, or even check the oil. might quiten it down.