Strange differential problem

[Rittmeister]

Did a LOT of searching here and on FA but found no answers.

I just installed a J30 diff into my S13. The driveshaft moved forward with no problem, allowing the fitment despite the diff carrier being longer for the ABS sensor.

Here's the problem - above 25 mph, and when I'm on the gas, I hear a humming noise that wasn't there before. It seems to be coming from the rear. It's not dependent on which gear I'm in, or on boost. It's only there when I'm on the gas and the pitch/volume rise with speed.

I checked for exhaust leaks. The driveshaft still spins by hand with about the same effort as it did before with the open diff. Every connection is tight, that's been double and triple-checked. The new diff has 80w90 Valvoline in it. The car drives fine and I can feel the diff working so I don't think it's worn (it was taken out of a J30 with only 35k on the clock). It's just making this wierd noise now, and I'm concerned.

Any ideas, anyone?

[uno]

mine does it too, different differential but same problem. i think mine has a fluid level problem, i haven't had time to check it though. That and i've got cheap gear oil in it right now.

[DeatschWerks]

When you install the ABS VLSD with a non-ABS driveshaft you will compres the carrier bushing. Often this causes a vibration. And almost always it will cause an early death of your carier bearing. I bet if you swapped your driveshaft to an ABS driveshaft (shorter) the problem would go away or at least be less noticeable.

I got away with a J30 diff and non-ABS driveshaft for about a year. There was always a little noise, but it got really bad at about 10-11 months.

[Rittmeister]

Fair enough.

So the driveshaft comes apart in two places - right at the diff, and right behind the carrier bearing. Is it that section that's shorter, or the section between the tranny and the bearing?