Rear noise like front wheel hub bearings going bad

[Goldfishy]

My 99' GCL 2WD automatic tranny (198,000 miles) is making the noise in the rear section that's familiar with the same noise that happens when our front hub bearings are shot and need replacement. The noise could also be described as a "whirring" like gears running dry, but not grinding nor metal rubbing.

What I've done recently is (but not due to the recent noise): replaced the front wheel/hub bearings, new rear brakes and rotor, new front brakes. A full tranny oil and filter change about 12,000 miles ago.

The sound is noticable when accelerating and when there's constant speed, BUT it goes back to "normal" (no noise at all) when I left off the gas. It is not a loud sound, and more noticable with the windows up and sealed versus listening with the windows down.

I had someone else drive, while I sat in the middle seats and was noticing the area in the trunk area was noisy.

My suspect is: bad rear wheels bearings or ?? Anyone have something like this or give me some directions as to what else to look for?

Thanks!!

[spytearbite]

Sounds more like the ring and pinion. If it was a bearing, it would sing on the coast too. Sometimes you shift the car's weight to one side or off the load side of the bearing and you can tell it might become quiet in this manor.

But when you can hear it in the middle of the back seat and then when you lift, you are off the drive side of the ring. So you gas it up again and the pinion is touching the ring at a higher spot.

A simple example is take your finger, wet it, now spin it slow over the ring of the wine glass. Did you make that glass harmonically sing? Also notice how the pinion (your finger) is farther away from center and is now touching more to the edge and this can cause the ring and pinion to sing that high pitch when you are under load. But, when you lift, your finger is off the rim of the glass but just moving in a circle until you press your finger back down and spin or enter the gas pedal and push that make your rear sing like a wine glass is a foot plot way to high and is now singing sour grapes you need a ring and pinion rebuild.

I mean, if this is not rear wheel drive with a rear axle and a pumpkin cone, then I do not know what a GLC uses for a final drive. Huston, we have a problem!

[Goldfishy]

Okay, thanks! Let's assume that's the problem is it drivable on a 900 mile mostly freeway trip with no trailer attached?

[spytearbite]

If you dump the pumpkin oil and it is silver and all powder in gray matter, then yes, it is going to matter down the road. If you add new oil and cross your fingers? You are just feeding new oil to a wearing down part on part.

Meaning, rub both hands together. Which one stayed cold? So, both parts are going to wear more or less equally or say one can no longer take the abuse and she goes south; you at 65mph is a fuse about to become louder before she seizes and costs you even more parts she about to take out.

So, to answer how long? Mark your Speedo now... Then, report back say you lasted a few hundred to a few thousand miles no prob is as good a guess as you.

For 900 miles of tow, I would dump the rear end oil and see how much metal debris is at the bottom? Then if it is somewhat clean, all you are hearing is that foot print a little higher in the (ring-to-pinion) grind your drone is turn the radio louder.

[Goldfishy]

I really appeciate your insights and explanations!! Especially to a guy like me, who likes things in simple terms, lol.....

Okay, I'm going to change out the rear diff oil with 75-140W synthetic gear oil (per owner's manual) soon and hope for the best for the time being.

Hope our family mini vacation does not end up with a worse situation. I will track my miles and report back, since I heard the noise at about 17 miles ago.

EDIT: Oh, now I know what you mean by "pumpkin oil".......I just happen to see that orange stain on the right half from top to bottom of the rear diff, but it looks like it's been baked on (all dried up).

[knelleken]

if it is a limited slip rear diff you will need friction modifier along with 75-140 synthetic. if you do not use the friction modifier you will burn up the clutches. also if it just whines or hawls and does'nt clunk,bounce(after letting off the gas) or feel as if its skiping a tooth, then your fine for your road trip. just dont do any burn outs along the way.

[jonmav30]

Ive got the exact same noise in my rear end and from what Ive found it is most likely a worn pinon shim so the backlash is slightly off. Ive personally driven over 20k miles with the noise, with many 500+ mile trips, and the previous owner said it had done it for at least the previous 20k or so, and Ive had no problems at all with it, and no metal shaving or anything in the diff fluid. As long as the noise doesn't get worse(louder) and you have no metal in the diff fluid you should be alright.

[Goldfishy]

As promised I'd report back with my updates: coming back after 1,238 miles logged on, we survived the issue at hand. While it has not gotten any louder, the noise is still there.

That evening before taking off, I did open the rear diff plug, fluid did leak out, it was light chocolate brown, did not see any gray color or metal filings. I did put back in synthetic 75-140W to top off. I don't have the limited rear slip diff.

I guess another way to describe the noise is like a brake rotor scraping the rear pads at near rivets (but again, those were just replaced pads and one new rotor).

Hope it doesn't get any worse this week and will have a mechanic look deeper into possible repairs.

Thanks all for your comments. Hope you all have a great Labor Day weekend.

[knelleken]

did you have the noise before your rear brake job? if not, you could have a spring clip rubbing on the rotor.

if you have excess to a jack and stands, jack the rear axle and place it on stands. have a friend turn the car on and stick it in drive MAKE SURE IT IS IN 2WD MODE rev the car up in-tell you can here the noise. get under the rear and listen to where the noise is coming from. if it is in the differential, take a large screwdriver (or stethoscope) and place the handle against your ear. place the blade end on different spots of the differential to try and pinpoint the exact location of the noise.
Note- your abs light might come on as a result of the different wheel speeds, abs will activate if you hit the brakes. please be careful of moving parts and hot exhaust. it will be safer if you lift the entire vehicle (all 4 wheels off the ground) due to it being a 4wd vehicle.

[Goldfishy]

Updating the current situation since the last comments, I did prop up the wheels as mentioned, the noise is coming from the rear end, and highly suspect the ring and pinion gear (maybe bearings too?) The noise itself has not gotten any louder (nor quieter).

In any hoo, does anyone know what an estimated repair bill may run, assume it's just the items mentioned above? Or since they have to open it up, what other parts should be replaced?

I don't want to get into a bill like some I've seen paying over $600 for a water pump job with a few extras, and didn't get a radiator with it (I know it's not the same mechanic problem, but you understand what I'm saying).