Rear End Noise and It's Not IBS. What Is It?

Is the rear diff or bearings?

The noise is a whining sound that starts at low speed (about 20mph), increases at about 40mph and tapers off above 55-60mph but is still noticeable.

I'm leaning towards the diff going bad because I can get occasional, but irregular, minor chatter when making a left turn from a full stop. Some seat-of-the-pants observations are that I also feel some minor "binding" when in a left hand turn.

What say the hive? Any advice on what to look for or expect before major repairs are required?

Oh, the vehicle in question is a '96 GMC S15 (auto/4wd) with about 96k miles on the clock. A long drive is planned for the near future and this is something I'd prefer to address before something happens on a road trip.

Thanks in advance!

Based on history, its likely eating the front pinion bearing. A gear whine will generally be much more pronounced either on drive or coast, while a bearing will be similar on either. If you catch it before the pinion actually starts moving around, the gears will likely survive and be quiet...or at least as quiet as a 96 S truck needs to be.

Take it apart, keep very close track of all the shims and their relative location, clean the housing as carefully as you can, including the axle tubes, install new bearings and drive. You shouldn't have to reset anything. The new bearings will be within microns of what the old ones were, so there is generally no benefit to concerning yourself with gear pattern.

Axle or side bearings can fail too, but its a different frequency. The pinion bearings will be turning 3-4 times as fast as an axle, so will make a much higher pitched whine.

In reply to Streetwiseguy:

Thanks for the input, SWG.

Anyone else agree or have other ideas?

sounds plausible . You will have to disassemble the differential anyway.

So a complete inspection is in order.

Definitely pinion bearings. It changes in tone with throttle correct? There's a sweet spot where it hums louder?

Grab the pinion and see if there's any play. Also check fluid level. It will hum long before play is perceptible.

On a GM 10 bolt you'll need a new crush washer and pinion seal in addition to bearings.

The only thing I somewhat disagree with is that you can just replace bearings and everything be fine... You do want to make sure backlash is in spec and make sure the gear pattern is right. I would get bearings from the dealer/same OE bearing maker if you're going to hope for the best results.

I've done a lot of regearing and gear setups, its pretty rare to replace a bearing or race and have the backlash not change in my experience. Admittedly I've done just a few GM axles, nearly all were Dana, Chrysler, Toyota or AAM axles. And front of a Mercedes R350 that supposedly couldn't be serviced. A common bearing(cross referenced to some 80s Benz wheel bearing) and a $200 bearing race from France that "doesn't exist" in the USA and a month after customs, it was good to go. Saved a customer $1800+.

FWIW look into a used complete rear end - they're dirt cheap and may be better/easier to do.

Edit - chattering while turning is likely LSD clutches. Try just changing the fluid(get the additive too!).

Or replace the whole assembly.

Randy's will have a rebuild "Kit" http://www.ringpinion.com/ if one bearing is toast replace them all that crud has floated around and contaminated the rest. Clean clean clean so clean you no longer smell gear lube in the housing.

Differentials never make noise in a straight line. They are internally stationary.

Probably carrier bearings. They weem to wear the fastest on the 10-bolts. Pinion bearings are much more load dependent. Mazda 7" for example seem to eat the front pinion bearing first, noted by the rearend getting very loud under engine braking.

Huh ? in a straight line, the pinion is turning, the ring gear and carrier are turning. the only thing that might not be turning is the spider gears but most likely they are too, albeit slowly. And they can make noise.

an interesting thing about bearings, with the same number it is essentially the same as another with that number. Of course quality of manufacturing comes to play. Oh, they are all metric

The gears and the bearings are not part of the differential.