I’ve searched for roughly an hour through this form and a few others via Google, and haven’t found anything quite the same in regards to my issue. So I have an older JKU 6 speed. Recently replaced an axle housing, not sure if that’s pertinent, and maybe a few thousand miles later I now have a high pitched whine that comes only when I accelerate in gear, when I take my foot off the gas it disappears, and when downshifting it is minimal. Almost sounds the same as when you reverse with any standard transmission. As far as I know it happens in all gears, but once I get mid rpm in 4th up I don’t notice it likely to wind noise. Any tips are appreciated, or if I missed a thread that answered this.

 

When i think of high-pitched whines that correspond with the speeds and driveline, my mind immediately goes to pinion bearings. Did you swap in an assembled axle, did you replace the housing only and sway your stuff in, or was that a completely new regear job? Certainly can't say for sure that is what it is, but that is the first thing that comes to my mind.

 

I changed one of the internal shafts in left side because it was bent and some new pinion gear assembly. I guess I’ll check there first, maybe they didn’t add enough gear oil or there’s a leak.

 

Well, if it is the pinion bearings whining, they are likely toast. If the preload on those via the crush sleeve and pinion nut isn't right, the bearings can crap out pretty quickly. If you've ever had a wheel bearing go bad, or even bearings in a serpentine belt pulley go bad, think about how easily those spin around with a simple flip of the finger, and the sound they make, relative to new bearings. That would not be a gear oil issue at all. The tricky part when it comes to something sounds from the underbody, or axles, is determining the specific location of the noise while driving around. If you had an old phone that you could rig up under the jeep and record the sound while you do a short drive, maybe that could be useful (ie, record while the phone is located right next to the suspected source).