Wondering if anyone knows what is the exact cause of my diff whine that occurs only under load. when at cruise its fine but as soon as you put load on it sounds kinda like a rear mount supercharger (not that they exist).

the diff is solid, and the gears are skyline r31 3.89:1 ratio, with all new bearings throughout the diff.

I used penrite limslip 90-180 weight which is for 4wd and commercial-- it was F**k thiiiick! would going to regular limslip oil of a thinner variety help quieten things down a bit? or is it just a case of curse of the skyline gears?