Screaming Noise From Rear End?

I have a 90 XJ, 4.0 AX-15, D44 rear.

The rear has been completely rebuilt, I'm running 4.10's and there's an Aussie Locker in there.

I recently switched back to a stock muffler and the other day I was on the highway and I thought I heard a low jet but the noise was coming from the rear end.

The noise only happens when I'm in gear and barely on the gas, not accelerating nor decelerating but just giving it enough gas to have no effect on speed. Sometimes it's louder than others, it ranges from a hum to a scream.

I opened up the diff, everything looked good.

Any ideas here folks?

New, or used, gears? What brand? Used gears are known for noise following setup, as are some of the cheaper gearsets such as Genuwhine, etc.

Who did the setup? Did they get a great pattern, a good one, or a so-so pattern?

The gears are Mopars, brand new. The guy who built the rear end works as a tech for the dealership and he built it for himself, then sold it to me. I have maybe 30K on this axle already, there are no vibes just noise.