As Agonair said, stright cut gears whine more than helical gears. My dad's '53 Oliver 77 had more gear whine than the newer tractors did. Trucks of that era really whined a lot more than the tractors did. Next time you are at a tractor show or a tractor pull listen to some Oliver tractors similar to your's, they all have some gear whine.
Your grandad's tractor probably had that whine when it was new, so to me anyway, it really would not be perfect if it didn't whine. Enjoy your grandad's tractor and have fun with it!