Thumping noise coming from rear

[yellowtrk]

My nephews 81 has this thumping noise coming from the rear when driving. It speeds up with more revolutions and slows down with less. I lifted the rear and it also did this with the tires off the gound. I removed the right rear tire, caliper and rotor and the noise stopped. Could this be the bearings or is it something in the differential? All the u-joints have been greased and seem real tight, no play. With the tires in the air there seems to be play if you push the top of the tire in towards the axle. There is nothing rubbing anywhere. This also did it after we had new tires installed and even swapped out rims. Thanks for all the help.

 

[Vettehead Mikey]

Probably a bad U joint, start there. Is the play in the rear wheel still there is you rock it at the 3-9 o'clock position or only at 6-12?

 

[LanceB]

Have you put the GM lube and additive in the differential? Mine was thumping before and that fixed it. Differential didn't have a drain plug so had to syphon all the old fluid out...

 

[yellowtrk]

It moves from the 12 to 6 position but I have not tried from 3 to 9. The u-joints seem fine, no play at all.

 

[yellowtrk]

Have not done anything with the diff. I'll have to check it out.