12 bolt rear diff inspection ?

[blazer2007]

Trying to find a resonating whirling noise at 30 mph. There is some noise but mostly at 30. U joints check out, no wobble on pinion. I,m going to open the rear and have a look but not sure what to look for except the obvious. I see on u tube the pinion doesn,t move that much as I don,t have a dial indicator. I can clean it and and try to see a wear pattern with some of that gear paint. Any suggestions or tips. How can I check the wheel bearings, I,ve checked for heat after driving but nothing. Is on a 1980 half ton 12 bolt open diff. Rear end was rebuilt about 3 years ago but I was never happy with it,mechanic said the Motive gears [3.42 gears]make noise but now this whirling noise is happening