I've searched and found some related threads with good info but nothing exactly like my concern. Most are related to 3rd Gens, probably due to age/mileage, but nothing in 4th Gens so mine may be unique.

Stock drivetrain with better than regular interval gear oil changes with Mobil synthetic 75W90. Slight amount of towing. Infrequent short runs with a utility trailer and a yearly long trip with a very light weight travel trailer. No apparent leaks and I've verified the fluid is at the correct level at the fill hole.

I'm hearing a whine while on throttle (manually or with cruise control.) The noise starts at ~53 mph (85 km/h), increases in volume with increased speed and then tapers off at ~60 mph (96 km/h).

As suggested in other threads, I've experimented with the following:
- The whine stops if I cut throttle or am on cruise and coasting downhill (no load situation) in that same speed range.
- The whine is still present in exactly the same speed range regardless of engine rpm; happening in 3rd and 4th (OD) gear. Rules out engine or tranny.
- No change in sound with side to side movement in that speed range. Rules out wheel bearing noise.

Still need to check lateral movement in U-joints.

I've only recently started to hear this to the point where I thought "what is that sound". I don't usually drive in that speed range so I'm not sure when it started and I'll need to remember to monitor it. I'm assuming it is going to keep progressing.

Am I right in assuming the main culprit is the differential? If so, is it the gears or the bearings? Is there an additive I can use to prolong the time to inevitable R&R?

The top 'gear head' (pun intended) around here is ZUK. He runs the website gearinstalls.com and does quite a bit of gear work for members on various sites. I've emailed him with questions and he graciously answered them for me.

You can email him at [email protected]

He would probably be the best guy to ask no sight seen/heard advise from on here.

Good luck.

Thanks for the info.

@ZUK , what do you think?

Just a follow-up.

Crawled under the truck to check the U-joints. They are tight with no vertical or horizontal movement, just a slight rotation.

Quick search on the net, I found references to using a heavier weight oil such as Redline Heavy Shockproof. I also found references to tightening the pinion nut slightly to increase the preload but this wasn't specific to a Toyota. What do you think of these ideas?

I'll keep researching this a little more to educate myself before taking to a driveline shop. Anyone else experience this?

don't tighten the pinion nut unless you have removed the third member and can check the preload on the bearings. You have to set that with the carrier removed and just the pinion installed.

Try switching to lucas 85-140 its a much heavier weight gear oil and see if that helps before tearing into a rebuild job. It's either the bearings wearing out or the backlash of the ring and pinion isn't set properly.

The whine is noticeable to me but can be drowned out by the radio volume. There are no other diff noises at other speeds so it may be early in the wear process.

It's (freezing) cold here so I did a fresh start run. The whine is still there in the same speed range with a cold diff. My immediate next step will be to swap out the gear lube when I find time. I'll have 18 months, 48,000 Km, 30,000 miles, with next to no towing on the Mobil synthetic so it may seem early. I'm thinking of just going back to 80w90 OEM dino as an experiment.

As for the pinion nut. Please see the attached pages from the FSM about changing the pinion oil seal. It is possible to measure the preload of the backlash while the third member is on the vehicle. I'll only do Steps 2, 3(a) and 14 to inspect the preload. I'll only consider carefully tightening the nut slightly if I find zero preload and even then I'll think twice and seek advice before doing it.

I know my limits. I don't have the expertise and tools to do a diff rebuild so I would leave that to my trusted mechanic or someone he recommends. I'm just researching to see if I can prolong or avoid going that route.

@BlackWorksInc , your thoughts?

Edit: added attachment.

Attached Images

29-44 x040001.pdf (75.9 KB, 1580 views)

lol for starters, there isn't a "preload of the backlash" you have a preload on your bearings and a backlash in between the gear teeth as they mesh.

The preload on the pinion bearings (which is what i suspect to be causing the noise) is set by tightening your pinion nut in effect adding pressure to the pinion crush sleeve. to set the preload properly you need to have the ring gear carrier removed so that the pinion gear will spin without any other friction than just that of the set preload on the bearings. it is then you can measure how much rotational force it takes to spin the pinion shaft. you tighten the pinion nut until it takes the required amount of rotational force to turn the pinion shaft. (basically it shouldn't be able to spin without something turning it and should stop spinning when you stop turning it or take your hand away if that makes since). I suspect in your case the pinion bearings are wearing out in which case the preload of the bearings has decreased. A heavier weight oil can give you a longer life on the wear that is already there but it will not stop the problem at some point the diff needs rebuilding.

to set the backlash of the gear teeth when re installing the carrier there are two carrier bearing caps that are threaded. you tighten/loosen these until your back lash is set in the right place, as you set your backlash you also set the preload on the carrier bearings. its a very delicate process even the most skilled mechanics can mess up. I've seen quit a few FJ's rear ends going out on them (pinion bearings failing) because they were not set correctly at the factory.


as for your fluid.. 80w 90 and 75w 90 are all going to do the same thing i dont care which brand you say is best better most awesome.. the weight of the oil is the weight of the oil. the differential is an open center diff by factory and has no need for any special fluids. as well with you being in cold weather freezing temps your fluid is going to stay on the lower temperatures for a long time before the axle actually heats up enough to vary the weight of the fluid. going to an 80w 90, straight 90w, or an 85w 140 is going to be your best bet. I recommend 85w 140,l its a much heavier duty oil.

all in all I definitely hope you get it worked out buddy

Thanks for the info. It reinforces what I've read elsewhere.

I'm going on the assumption that any change in noise (a new whine) isn't a good thing and I'll eventually need the diff worked on. I'm just going to do the gear lube change for now and monitor the situation.

thats what i would do, not sure if you've priced diff work or not but there is a company here in northcarolina called Drivetrain Specialists East Coast Gear Supply they will build a complete differential ready to be intsalled and give you a five year warranty on the ring and pinion setup (basically guaranteeing the backlash is properly set). you can have a stock third member built and shipped to you for 650, ready to bolt in so you could manage it yourself. Not sure if that is a price advantage for you or not you'd probably have to quote around. But they ship you the new one and then you ship your old one back in a three week window to avoid a core charge.

Thanks again. I've seen ECGS talked about in other threads.
I haven't talked to my mechanic yet or shopped around. I've had small items shipped to MI and then I pick them up. This is bigger and has a timeline. Shipping costs across the border may be a deal breaker.

Finally got some time and swapped out the existing Mobil 75w90 with Shell Spirax 80w90. Old lube had 46,600 Km (29,000 miles) / 17 months / 972 running hours on it. There is a slight, marginal change. The whine is still there at the same "pitch" in the same speed range but is not as loud. I'll monitor it until spring and decide what to do depending on how it progresses.

Another update:
Over 2 weeks & ~800 miles since the gear lube change and the noise is considerably diminished. Not as loud, lower pitch and over a shorter range. I know this isn't a fix but I'll keep an ear on it for now.

So is this differential problem - a known issue, or are these just unique occurrences that came improperly adjusted from the factory? We have a 2014 Trail and I would like to know if this is something to be concerned with.

Ok so I am noticing the same noise. Medium pitched whine when I hit the accelerator. I just changed the diff/t-case fluids about 6 months ago. can't remember if there was a noise before. Truck has a 170k on the odo. the diff fluid has been changed twice in its life.

Question is, if I ignore this issue what will happen?

Edit: If I need a new 3rd member, I guess paying the extra dough for an elocker is in order?

This is not a problem on new equipment, unless the equipment was assembled incorrectly. This is an issue that develops over time as the bearings wear out. 150,000+ mileage (even that's on the low end)

It's not quite spelled out in my earlier post. So in short, to fix the whine. tighten the pinion nut. This increases the preload on the bearing and stops the whine. Factory diff has a crush sleave installed so as the bearing wears you simply crush the sleeve more and tighten the bearing back up. Tighten it to much and you can cause to much preload pre-maturely wearing out the pinion bearing. Don't tighten it enough and well you have the same problem.
If the pinion bearing is worn past the point of adjustment with the crush sleeve you'll need to replace the pinion bearing.


If you do nothing the whine will simply get louder over time at some point the sloppiness of the bearing that the pinion is riding on will likely cause leaks in the pinion seal and diff fluid will begin to leak. This can also increase the likelyhood of pinion deflection in high torque yield situations. This would be worse case scenario.

The cost of an e-locker vs. rebuilding a diff is not comparable. Also if you install an e-locker it is advised to rebuild that elocked 3rd member so that it includes a solid pinion spacer vs. a crush sleeve. Crush sleeves allow pinion deflection at high torque yields, which cause broken teeth on ring gears. Also elockers do not directly bolt into non elocked housings. You must cut a notch in the axle housing, drill and tap two new stud hole locations. Its doable and arguably a better solution than an Air locker as you already have on board electricity.