12 bolt rear diff inspection ?
 

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

Re: 12 bolt rear diff inspection ?
 

Is it a high pitch like a whine (pinion bearings) or more like a growl (axle or case bearings)?

If you take the cover off, with a prybar, try to force the differential case (carrier to some) left and right. There should be no play.

You might have to pull the axles to check the axle bearings. Sometimes a mechanic stethoscope can be helpful but the axle tubes will transmit sound down to each end.

Re: 12 bolt rear diff inspection ?
 

It,s a whine like no oil in the diff. and hits 30 mph and screams in the truck and then goes away at 32 mph. I hit a local garage who I see has a hot rod and he did the stethoscope on the the tire bearings and said not bad but some wearing. rear sounded not bad in the air with the rear running but he said these things are like a tin can for resonating.