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I have a 96 GMC 1/2 ton P/U. 2 wheel drive, 2 part drive shaft. 350 Engine, 4 speed Auto trans, 10 bolt rear end, 8 1/2" ring gear. I am hearing a kind of rumbling from the drive train. Not steady, but like something is rubbing on part of a revolution. Noise starts at about 30 MPH, does not go away, cycle of noise increases with vehicle speed, in or out of gear. No noise on deceleration, or with "hard" braking while at steady speed. Have done this: 1) Replaced rear wheel bearings and seals 2) Changed differential oil (Castro Synthetic 75-90) Inspected gears 3) Replaced "U" joints 4) Re-packed front wheel bearings. 5) Checked tires and wheels, no contact anywhere 6) Cleaned and inspected rear brakes. No obvious sign of a shoe dragging, The only thing I didn't replace was the drive shaft center support bearing, which turned smoothly and rubber casing was in good condition. Test drove - No vibration up to 90 MPH. Still have the noise. Any ideas, or something I may have overlooked?

The noise you are describing sounds a lot like the carrier bearings in the rear end. They will make a growling noise at all speeds and eventually will hurt your ring and pinion gears if left unattended. To verify problem you must disassemble the rear end and inspect. So pull the rear end cover and remove the axles. After this has been done remove the 4 bolts that hold the ring and spider gear assembly. The carrier bearing and races will come out with this assembly. Look at the bearing races and I bet you will see your noise problem. (They should be pitted and rough looking if I am right) Buy ONLY original equipment bearing from the dealership and have a machine shop press the old ones off and press the new ones on. Reinstall the ring gear assembly and TORQUE these bolts to the proper specification, replace axles and fill with new grease. Let me know what happens when you take it apart for your inspection.

 

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