I would recommend firstly opening the box up and checking it out, maybe it could be bearings or rings needing replacement if the gears are in good condition theirs no need to buy a new one.
I had certain problems with my gearbox not to long ago, it was doing very bad whining noises and crunches on 3rd and disgusting other sounds when in any gear and off the gass pedal, opened it up and it looked mint, found out one of the bearings where gone and the snap ring needed replacing, put it all together and it sorted out everything, whining noises still continued then I found out it was coming from the propshaft because the diff I have battered the shaft joints. That's my recommendation.