drive train rubbing type noise proportional to speed

This noise starts at about 30 mph ( 2 cycles per second (cps)) and increases to 4 cps at 60 mph and sounds like it is coming from the rear. It is there on engine power and drag and 2 and 4wd. It started about 3 months ago and has been getting progressively louder. The closest description I can give is the noise is similar to what I would expect from a tire momentarily rubbing on something under the vehicle. I did look and there is no such contact. I ran a test on my hoist with the wheels off the ground in 2wd only and the noise was still there. During this test both rear wheels were turning. An interesting statistic is that at 60 mph the wheels are turning at 12 rps so I have ruled out brake and tire issues. The rear diff oil is due to be changed at fall service and the level is good. Anyone have any ideas as to the cause.

 

I've read a few threads about people changing out their gear oil for new and adding extra limited slip additive to the oil and it quiets down noises. May be the easiest thing to start with if the noise has anything to do with the differential.

 

Thanks for the input. I have had a dealer install the wrong oil in the past with terrible driveability consequences on turns. They even tried to charge me for removing the wrong oil I had paid them for and charge me again for the new stuff...didn't get away with it. I believe that there is a sticky dealing with the wrong oil consequence. My concern now, due to the age of the vehicle, is where the cost to cure the noise is going to lie within the price range of a wheelbarrow thru to a new truck. My plan is to take the truck in for service, find the noise, change fluids, check the noise again and move on from there. I need to solve this cause first because the truck is due for a new timing belt and radiator with the possible resulting damage to the a/c hardware attached.

 

I have what seems to be the exact same noise. Keep us posted on your results and I'm off to have my diff fluid changed.

 

Changing the LSD oil made no change in the problem. I have 3 possible culprits on the horizon, bad axle bearings, bad lsd differential and one or both of these interacting with the ABS system, hence proportional to speed. I hope to open the thing up next week and learn more but will disable the ABS and test before taking things apart. At 60 mph the noise is 4/sec and at 30 mph 2/sec and nothing else is turning at that rate, e.g. drive shaft about 44 and 22 rps and axle at 12 and 6 rps so I wonder about ABS and computer control. If I learn anything worthwhile I'll report.

 

is there a bearing at the slip yoke connection that could make this noise??

 

Drive a constant 60 mph until you hear the 4/sec noise, then disengage the overdrive. The engine RPM will increase while you hold a constant 60 mph. If the 4/sec noise remains the same, then you can eliminate transmission and transfer case. If the noise changes frequency, however, this suggests that those components are involved.

 

Thanks for the new input. I suspect that any rear end bearing noise would by itself be too high in frequency for what I'm hearing but may interact with the LSD and maybe ABS to cause this symptom. I can confirm that the noise source lies past the transmission. The tests I ran included disengaging the overdrive at 60 mph and running down sequentially to second gear (30 mph for that one with a 2/sec noise) with no observed changes in frequency of the noise. I won't be convinced that ABS is not involved until I shut that off and retest. If I have to get into the differential the loss of a speed sensor will be a small sacrifice for knowledge gained. More to come.

 

I'm totally lost as to how the ABS could cause your problem, or how disabling it would change a single moving part on your rear end. The tone rings are not physically connected to the ABS system, and cutting power to the speed sensor won't create/resolve any sounds. Do you suspect the fluid is pulsing voluntarily from the pump while you drive with your foot off the brake? And if so, wouldn't you hear it in the front? Or feel it braking?

 

What about an exhaust leak?

 

Definitely not an exhaust leak. The ABS logic follows. The ABS controller operates on each wheel independently using momentary lockup seen by a speed sensor to release the brake on that wheel and reapply it. Here is where the "what-if"comes into play. With the noise under discussion at a wheel speed of about 12 rps the ABS controller (healthy or otherwise) would have to be operating every 3 wheel rotations over an unknown number of degrees of rotation. My line of thought for this is could a slight periodic braking force be enough to fool the LSD into periodic operation keyed to a wheel sensor speed output? The brakes seem to operate normally, there is no sensation on the brake pedal and no ABS warning light. Disconnecting a single sensor and doing a test run on the hoist seems like a worthwhille test before getting into the oily stuff. I guess I could pull fuses and make a bunch of other things happen at the same time. Thanks for your interest.

 

Thats some pretty wizardly diagnostics, my mind is blown as my truck doesn't have magic independent wheel braking. I have a luddites non vsc, non lsd. None of my lsd vehicles have ever been audible when engaging.

 

I think someone put a playing card in your spokes.

 

This was my 1st thought when I read this post not long after OP made it, but opted to keep my mouth shut because I seem to be gaining a reputation for ribbing too hard on the newbies who sign up to post their problems without any meaningful introduction.

 

Its not ribbing, its evidence of my level of understanding, but I could be wrong, might be a banana in his tailpipe causing a toot, synchronous with the pressure pulses but only under load.

 

Got an automotive stethescope? Have someone spin it up while on the hoist and pinpoint where the noise is coming from.

I'm guessing ring and pinion/3rd member.

 

if you calculated your axle to be spinning at 12rps, and your rear diff is geared at 3.x, your driveshaft is spinning at just under 4 rps.

EDIT: DOH!!! False backwards math. Nevermind. carry on.

 

if you think your lsd is locking multiple times per second, your truck won't behave well on turns. Do 60 mph full lock circles, let us know if you get wheel hop.

 

Check the emergency brake pads. They are inside the hub of the rear brake rotors. One of the pads may have shifted inside and rotational force at speeds is generating a rub.

 

are those like maxipads? we don't have maxipads or tampons in first gen rear brakes.

 

Definitely don't want to rub one out.

 

Maybe its your douche bag dragging. Good luck figuring it out then!

 

If you came to be helpful, just point him to the page with the diagram of the rear disc assembly of his 2005.

 

Perhaps this...

https://img.ccjdigital.com/files/ba...12-45.png?auto=format,compress&fit=max&w=1280

 

Sounds like a bearing. You've got carrier, rear pinion, and outboard wheel bearings as likely culprits. Carrier can be inspected visually for play and noise with the rear wheels off the ground. Pinion and outboard wheel bearings may be isolated with an auto-stethoscope. Good luck with this.