Outer Limits 4x4 Board

recently I have noticed a strange hissing sound coming from what I believe is the gearbox in my triton. It is really noticable in 5th especially at speeds above 80ks. The sound is gone when you are coasting or when you change gear. I thought that it might caused by damage to the exhaust but it is fine. Car drives fine and gear changes feel the same? Any ideas what may be causing this- I'm thinking that 5th gear maybe on the way out?

I'm not sure what model yours is but i do know that the Tritons had drama's with the bearings in their gearboxes in around the mid 90's models to current.