Outer Limits 4x4 Board

I think this is called transmission whine. Whenever I am in 3rd, 4th or 5th theres horrid whinning noises comming from the gearbox. The gearbox is an R151F. I do have a spare box.

Any remedies?

Drop the oil and check for metal bits. ONLY if no oil comes out or it's clean, re-fil it and try again, drive it and see how it goes.

If you got metal bits, don't waste new oil, change the box.