I would re-check your preload on the pinion bearings. I learned the hard way about deburring shims on a 12 bolt I rebuilt. The pinion bearing lost some of it's preload after a couple hard runs and gears started whining and I pulled back apart and reset the pinion preload and re-checked the pattern and it still looked good. Put it back in the Firebird and we lucked out it was quiet again. The only thing I could come up with was the stack of shims under the pinion bearing cup had edges on them that gave me a false reading. Just a thought
Gus