Driveline Noise when Decelerating

NTL1991

MEMBER
1997 CHEVROLET S-10
6 CYL
4WD
AUTOMATIC
174,000 MILES

I have a driveline noise when decelerating from ~35MPH and above. When I am driving along and I take my foot off the accelerator, once the RPMs drop to around 1000-1200, I will get a rumbling/grinding noise from the center/rear of the truck. It resembles the sound a playing card makes in bicycle spokes, but a bit deeper. If I just TAP on the gas to raise the engine speed even just 50RPMs, the noise will go away. I get no noise when accelerating or when maintaining a higher engine speed. I have noticed that the activation RPM increases with vehicle speed. For example, at 55MPH, the sound appears when the engine is at about 1250-1500RPM or below, but not above that.

I have the G80 Posi-Traction Rear End which has brand new LSD-Approved oil in it.
The transmission shifts fine with no flaws.
The Universal Joint was replaced about 3K miles ago.
This only happens when the car is moving.
This problem has only revealed itself a few days ago.

4 Replies

DOCFIXIT

MECHANIC
18,828 POSTS

Give the catalytic convertor and inspection. Bang on it if you hear rattling then that maybe the noise. Engine decell causes a great deal of back pressure.
Check it out let us know.

Pinion angle more likely the cause. accelerate and de-cel can cause a slight rotation of the rear end. When this happens the pinion angle changes- Too flat or too much angle can cause vibration.

Also check the differential fluid.

https://youtu.be/v6PjkozhrAc

Please let us know what you find.

Cheers

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Tuesday, November 4th, 2008 AT 11:36 AM

NTL1991

MEMBER
16 POSTS

Thanks for the fast reply. I actually brought the truck into school (I'm still in High School) and had my auto teacher do a delta-T test on the catalytic converter and the exhaust temp only increased 5 degrees when traveling through the cat rather than 100 it should have. When I hit the cat with something metal but I get a glass-like sound. No rattling, but it also doesn't sound hollow.

Thanks,
Nick

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Tuesday, November 4th, 2008 AT 11:59 AM

NTL1991

MEMBER
16 POSTS

Well, Unfortunately, I figured out my problem the hard way. I decided to drive the truck on the highway to see if/when the noise occurred at highway speeds (70 or so) and it was doing the same thing at slightly higher RPMs. I decided to take the quick "U-Turn" exit on the left of the highway so I would be going northbound back home. The U-Turn requires that you come to a stop and traffic was horrible at this time, about 4:30PM, so I saw a break in traffic stomped on it and BOOM! The truck wouldn't move forward in gear so I popped it in 4HI and pulled to truck to the left side of the highway. At first, I thought I blew the transmission but its fine in 4-Wheel Drive and the driveshaft isn't hanging down or anything so my uninformed guess is that my Posi-Trac 3.73 Differential is toast... When in 2HI and in Drive, I get a whirring sound and when I put it in park or change the gear an awful clicking sound is heard then stops...

So the truck has been towed to the shop that put the remanufactured engine in... This truck is getting expensive. Over $3800 in parts/repair alone... But I still love it.

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+1

Tuesday, November 4th, 2008 AT 5:30 PM

DOCFIXIT

MECHANIC
18,828 POSTS

Well you found the problem. Sometimes you got to break it to fix it. It is a fun vehicle to drive. What about a project for auto shop?
Makes clicking sound cause D shaft is still spinning and your hearing the park pawl try to lodk trans.

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Wednesday, November 5th, 2008 AT 12:50 PM