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Rear clunking?

Tiny
DOUG1313
  • MEMBER
  • 1996 BUICK ROADMASTER
  • V8
  • 2WD
  • AUTOMATIC
  • 76,000 MILES
This is a wagon with positraction. Occasionally at speeds less than 3mph with drivers window open I hear a clunking noise coming from the left rear of the vehicle. I had a 1994 that did the same thing and the rear end needed to be replaced. Have had brakes and rear differential inspected both OK and had differential oil replaced with posi additive. Ideas?
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Wednesday, December 3rd, 2008 AT 1:23 PM

7 Replies

Tiny
JAMES W.
  • MECHANIC
  • 2,394 POSTS
A couple questions. Is the "clunk" at wheel speed or faster? Does it clunk on acceleration, deceleration or both? With the vehicle stopped, does it clunk when shifting from drive to reverse and back? Please advise.
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Wednesday, December 3rd, 2008 AT 7:38 PM
Tiny
DOUG1313
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I would say clunk is at wheel speed. Only hear noise at walking speed or slower. Have heard it when slowing and after releasing the break, as the speed increase clunking stops. No clunking when shifting transmission.
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Thursday, December 4th, 2008 AT 9:02 AM
Tiny
JAMES W.
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When the noise is present, if you "lightly" step on the brake, does the noise cease? Also, when you step on the brake during a normal stop, does the pedal pulsate up and down?
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Thursday, December 4th, 2008 AT 9:43 AM
Tiny
DOUG1313
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Hard to answer your first question because it happens infrequently and I have not paid attention to that and the noise appears at such low speeds that when you step on the brake the car stops and the noise ceases. No there is no brake pedal pulsating in normal stops.
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Thursday, December 4th, 2008 AT 9:49 AM
Tiny
JAMES W.
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Well, the only thing left is the positraction itself. The noise you describe may the clutches in the rear end slipping just a little. This usually occurs when running two different sized tires on the rear. I'm sure your running numerically the same size tires, but the "physical" diameter may different just enough to cause the clutches to jump. You may want to check this. I used to supply tires to a guy with a circle track race car. He ran P235/15 tires on his car and would measure the inflated diameter of each tire. The diametwers would vary as much as 2 1/2" in. On brand new tires. He would run the larger ones on the right side. Let know what you find.
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Thursday, December 4th, 2008 AT 11:53 AM
Tiny
DOUG1313
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Thanks for your analysis. Is this something I can ignore at this time given its sporadic nature?
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Friday, December 5th, 2008 AT 8:37 AM
Tiny
JAMES W.
  • MECHANIC
  • 2,394 POSTS
I wouldn't consider it serious. GM positraction rearends have made noise for eons. It is considered charactoristic of the design, most commonly caused by the wrong additive or lack of. According to your posting, you have an additive so I wouldn't be too concerned.
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Friday, December 5th, 2008 AT 9:56 AM

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