Differential noise diagnosis

Hi everybody. I’ve got a noise coming from my 89 Bronco II RWD automatic which I’m having trouble diagnosing and I’m looking for thoughts on the matter. This post is a bit lengthy, but I want to make sure I include all relevant information.

So here’s the problem: I’m getting a high-frequency, low-amplitude whirring noise coming from somewhere in the truck. I believe it’s the differential, but I’m not totally certain. The noise occurs from about 44mph on up and is loudest around 65mph. It happens under moderate acceleration, cruising, and floating (backing off the throttle to decelerate without engine braking), but it does NOT happen while coasting (decelerating without throttle), and I cannot hear it under heavy acceleration. The noise is definitely related to wheel speed and not engine speed, and what gear the transmission is in doesn’t seem to make a difference.

At the beginning of last summer I needed to make a trip from Daytona Beach to Cleveland for an internship. I ended up towing a 900lb trailer (my motorcycle) plus about 100-200lbs of cargo in the back of the truck. Along the way, whenever I would hit a dip in the road or reach the bottom of the hill, I’d feel a momentary “extra” vibration, almost like a grind. I figured this had to be because of all the extra weight on the rear end and immediately regretted starting this trip. But at the time I didn’t have any other choice but to keep going and eventually made it to Cleveland in one piece.

I first noticed the noise about a month or so after that during a time when the truck had not been doing any heavy lifting. I can’t recall any one particular moment when it started, nor a specific singular event that might’ve triggered it. At the end of the summer I needed to make the trip back down to Daytona, and in preparation for the trip I topped off the oil in the differential (it was about half a quart low) and I got some helper springs to handle the load better. No more instances of “extra” vibration on this trip. However, the Appalachian Mountains proved to be too much this time and my baby broke down coming out of Morgantown, WV. Ended up having the transmission rebuilt and the water pump replaced, but I eventually made it home.

Since then I haven’t been able to do anything about it. The noise has persisted, but at least has not gotten any worse. I’ve also got a “clunk” sounding off whenever switching from forward to reverse and vise versa, which I think is due to worn u-joints in the driveshaft. I’ve got those CV u-joints and I’m thinking about swapping the shaft out for this:
http://bit.ly/2pp0DZG

I currently have the diff opened up to change the fluid. I discovered the tiniest of cracks on the mating surface of the stock fiberglass cover and now I’m waiting on a new steel cover in the mail. That explains why the bottom of my diff always looked wet but there was never any oil on the ground beneath it.

Based on the conditions and symptoms I’m hoping someone here might have some insight into what may be causing my noisy woes. I can inspect the differential but I just don’t know what to look for. I’m thinking worn gears or bearings…I’m just not sure. Any help that can be offered is much appreciated. Thank you for reading.

I know they're not the same but I have a 99 ranger with a Ford 8.8. I had replace my rear and front differential pinion seals and my truck made the same noise but only when I was coasting and no noise under power. After inspecting my work I found out I had about 1 maybe just a 1/2 mm of play on the driveshaft flange that's fastened to the pinion shaft by a large nut. Using a 1 1/16 inch socket, breaker bar and cheater bar this time and tightening the nut as much as I could(which allowed no more play. I drove it and haven't had a problem since.

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Hmm, that could be a possible solution. But can't tightening the pinion nut mess with the bearing preload?

id pull the cover off, my brother in law, had a toyota pickup that had a really loud noise coming from somewhere in the pickup, i assumed it was his shitty cheap mud terrains just howling on the highway. it ended up being the rear ring gear, one day in town it pickup up a tooth and stopped the rear end from turning.

maybe put your hand on the diff cover after driving it a bit, if its super hot, id pull the cover. probably do for a change in fluids anyways.

After doing a bit of reading on differential wear, and since I already had the cover off, I decided to examine the gears and everything as best I can.

Rather than trying to type out all my observations, I made this video and posted it on youtube:
https://youtu.be/rSACLXB4nAY

im curious to know why that passenger side spider gear doesnt always spin. i dont remember seeing that much free play in my spider gears when i super packed my 8.8, in fact im pretty positive that i had none.

if its 4x4 you could pull the rear drive line, and drive it around to make sure it is/isnt the rear end. like you said without a dial indicator its hard to tell backlash, but that doesnt look excessive to me. when i regeared the 60/14 i saw what excessive amounts was. but it could also be out of spec.

did you find many metal shavings when you pulled the cover?

i would say being a 3.73 ratio just go find a 8.8 373 ratio and swap it in. they are everywhere.

Make sure theres no play in the pinion, make sure theres no play in the carrier assembly, and check that the backlash is between .006-.010". If you can put it up on jackstands and have somebody operate the throttle (with the diff filled with fluid) and listen to the bearings with a stethoscope under different conditions. It will not be as loud since there is not very much load but if the bearings are bad enough theyll let ya know. Check the t case and transmission too, you will hear some noise but bad bearings make a grinding sound.


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