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Driveline Noise, 4WD issue

Sent to Car Experts January 12, 2006 8:15 p.m.

I own a 2001 Chevrolet Suburban, 4WD 1500 with 5.3 liter engine.
The truck has 38,500 miles, and I am searching for the source of a running gear noise. Based on past experience with a Chevrolet G20 van, I thought it was a rear axle bearing, but I have pulled both axles and they are fine. I used a mechanics stethescope to listen to all the bearings in the differential, and all are quiet. The only one which makes any noise at all is the pinion bearing, but it does not seem likely enough to cause what I am hearing. The sound is like a rotational grinding, which changes as speed increases.

Two question: Is it possible that a universal joint could cause this sound? Any other sources which might be the culprit? It does seem to be coming from the rear of the truck.

One more important issue: While working on the rear axle, I had all the wheels off of the ground, with the truck on stands. I wanted to listen to the driveline, so I ran it in D with the parking brake off, engine idling. To check the front bearings for noise, I put the truck into 4High. I noticed immediately a curious issue: The rotational speeds for the front and rear wheels were obviously drastically different! How is this possible? If there is a direct mechanical linkage via the transfer case, the disparity could only happen if there were unequal final drive ratios front and rear. Any idea what is going on?

Thanks,
Alan

Optional Information:
2001 Chevrolet Suburban1500 4W 5.3L

Already Tried:
Removed both rear axles and inspected. Bearing surfaces clean and blemish free. rear brakes are new, including the parking brake shoes, rotors, and pads.

Customer (name blocked for privacy)

Status: Closed Value: $10

Answer
January 12, 2006 8:43 p.m. (28 minutes and 17 seconds later)

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john -- Auto, ATV, Sleds, Bikes, Tractors -- 95% Positive Feedback on 285 Car Accepts
26 years of mechanics experience

Reply
Sent January 13, 2006 9:31 p.m. (1 day and 0 hours later)

The use of the stethescope has been extremely helpful in the past to locate a cause of noise. In this instance, none of the bearings, including the pinion bearing, make any more than the minimally expected amount as running gear does when in operation. The pinion bearing makes more than the others, but only by a very small degree. It does not sound "bad" to me. I have heard rough bearings, and this is not it. The sound does not "wow" or 'sing" like a bad ring/pinion will. I'm pretty much crossing off the entire rear axle as a source of the noise.

Regarding the appearance of different rotating speeds front to rear: The disparity is so great I cannot be just imagining it. I did not have an opportunity to do a count of drive shaft vs. wheel rotations for front and rear, as I was working alone. It is on my list of things to do next time I have the truck with 4 wheels off the ground, and have some help.

BTW, none of the links you posted in the reply work. I tried accessing through aol, and directly with IE, same result.

Customer (name blocked for privacy)

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