So about a year ago, I started to get a high pitch, but muted whine, which I think is coming from the rear end (front wheel hubs were replaced in 2017). It begins at about 50 mph and happens when my foot is on the gas. It's pretty low, to the point where someone not attuned to every sound on this truck as I am, would not notice it. I don't think it will implode anytime soon, but I'm preparing myself to do something about it.

The rear end is 9.25 open diff. Fluid is good, never ran low, never dirty. Gears looked good last time I checked, not much paste on the magnet and no metal pieces. That was 14K mi ago, when I replaced the fluid.

I've never had the pleasure of rebuilding one of these (never had to). Would the pinion bearing be a good guess as to the cause? I've come to understand that there are inner and outer pinion bearings. I'm assuming the inner is lubed by the gear oil and the other is not (just grease I guess)? Would it be the outer that usually fails? I understand that the outer is replaceable without having to pull the diff apart. The inner, being another story. Sound reasonable, or am I barking up the wrong tree? Gonna put it up on jack stands and run it to 50 mph soon.

 

I think pinion bearing would be a good bet. All the bearings are lubricated by the fluid in the diff. The ring gear tosses it around nicely, to make sure everything gets covered.

The outer pinion bearing can be changed without much fuss. Getting the race out with the pinion in place though, that can be fun. Probably wouldn't hurt to inspect the bearing/race, see if anything obvious presents itself.

 

Good to know. What are the chances of it also being an axle bearing? Those are also lubricated by the diff fluid, right? Guess I'll know when I run on the jack stands. Can't believe this is happening so soon. I mean, I've only got 323K on it.

 

Axle bearing would likely make noise all the time, or just around corners. Pinion bearings are only under load when you have your foot on the gas. So, coasting, it won't be noisy.