So no whine in 4th gear?
That tends to signal an issue with the mainshaft roller bearings between the input shaft and the mainshaft. These are the 15 or so loose bearings that the mainshaft snout sits in inside the input shaft.
The whine is usually the loudest in 1st and reverse due to the differential turning speeds between the two shafts being greatest. It will be quieter in 2nd and 3rd as the shafts get closer in speed...and silent in 4th due to being 1:1.
usually it's caused by low fluid level or improper fluid. The fluid doesn't get to the bearings and lubricate it properly and the shaft of the mainshaft scores up and you get the whine.
The fix usually requires replacing the mainshaft as the tip gets chewed up pretty well.
You can try changing the fluid or making sure it's topped off, or replacing the bearings, but inspect the tip of the mainshaft and if it looks damaged you'll need to live with the noise or replace the shaft
The damage will look something like this..which resembles pitting