A month ago a new noise showed up in 1985 GS. Without going into all the history, we replaced the differential fluid and the noise diminished. Then we replaced the manual trans fluid and the noise reduced more. U-joints are fine.

Now it is a high-pitched whine or howl that occurs only between 40 and 50 MPH, and then only in 4th and 5th gear when under power. Coasting or slowing down between 40-50 MPH in 4th or 5th gear and the noise stops.

I really can't tell whether it is coming from the diff or trans. What do you think? Throwout bearing? Other transmission bearings? Differential bearings? Adjustment of ring and pinion gear clearance in the differential?

 

I think you shouldn't be driving 40-50 mph in 5th gear for one thing.
But, all kidding aside, howling or whining sounds usually indicate worn gears, or possibly mainshaft bearings. These Mazda transmissions all seem to be a bit noisy to me anyway.
FWIW, I always replace the transmission and rearend oil with Amsoil synthetic 75W-90 gear lube, and it does quiet things down considerably more than even fresh conventional gear lube.