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What could be causing a popping / clunking sound from the rear end of a 1996 Caprice? The sound is only present (or apparent at least) after the vehicle has been driven for a while and seems to correlate with the brakes being gently applied or backing up. Anything I should check besides the differential and the rear drum brakes?

Update #1

I should mention the vehicle has a torque converter with a higher than stock stall speed that went in with a fresh 4L60E. There is no torque creep to speak of and there is a bit of a whiplash effect when accelerating from a stop with a heavy foot on the gas.

Update #2

The differential (G80 RPO) was low on fluid. It was leaking out of the pinion seal, which is a common problem on this generation of this platform. Fluid has been changed and the seal replaced. The noise has changed, but it's still there. Not as many pops/clunks, and the frequency has slowed, too. I wonder if the fluid was low enough long enough to damage the rear end? I suppose it could be age related, too. It's probably the original rear end, and it has over 150,000 miles on it. Front and rear brakes appear to be good.

The sound is present when letting off the brakes after a stop after driving at highway speeds.

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Is it a single clunk? –  Larry Apr 6 '12 at 17:33
    
No, it's a series that lasts for a couple seconds –  Mark Johnson Apr 9 '12 at 18:40
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4 Answers

The universal joints.If they are really bad you may be able to see signs of rust on them or if you are under the vehicle.You can also grip the drive shaft while the car is supported on jackstands and the rear tires off the ground and see if you can push/pull it and look for movement.If you twist the driveshaft by hand you may be able to feel the click in the bearings.

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It could also be the shocks - if the mounting points are loose, reversing with the brakes gently applied could cause them to rock backwards.

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It happens in drive, too. –  Mark Johnson Oct 2 '12 at 17:48
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I have similar symptoms when my sway bar bushings need lubrication. There's a distinct sensation at low speeds where dumps will cause relatively sedate deflections and rotations of the bars. The bushings will briefly bind, release, bind, release, etc. in fairly quick succession. It can sound like a pop - clunk if it's just once catch and release.

This one is pretty easy to check:

  1. Spray penetrating oil liberally on the sway bar bushing.
  2. Allow to soak in (some minutes).
  3. Drive to a location where you can normally reproduce the sound and give it a try.

If your bushings are worn out, they are relatively easy to replace.

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If it's a single click it may be the brakes. The self adjusters activate when the brakes are pressed when backing up, they are under spring tension and can make an audible clicking sound when they adjust. If the adjuster screw is seized the noise would be much louder as the adjuster rakes across the teeth of the screw.

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It happens in drive, too. –  Mark Johnson Apr 9 '12 at 18:43
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