So I tried to get my microphone to record directly to digital audio, but I found out phones are only audio inputs on the headphone jack, so I tired my laptop but I couldn't get it to work either. So I hooked the headphones up and turned the volume all the way up and recorded with my phone so there's a little bit of noise.
I hooked the microphone clamps up to all 4 sides of the diff, one on driver's side on the nut that holds the oil cooler fitting on, one on the passenger side of cast surface, one on the front near the pinion input on cast surface, and the other on the very back of the diff on cast surface.
Unfortunately I can't really tell which one is louder. I was hoping to try to figure out if it was a pinion bearing, or driver's or passenger side axle bearing.
Since I can't really tell I guess that means it might be the differential gears. To me it doesn't sound like a gear whine, but more like a grinding bearing sound. But the fact that it makes a completely different sound when you are on the gas versus coasting makes me think its a gear as well. I would think a bearing would make a constant noise weather on or off the gas, maybe a pinion bearing might change pitch a little but this is a completely different sound when on and off the gas.
I recorded one with just my phone inside the cab with no microphone hooked up but you couldn't really hear anything.
I also read through the service manual, apparently the only thing GM thinks is serviceable on these diffs are the seals, axle shafts, and gaskets. So they expect you to buy a whole new diff for everything else I guess.
Diff noises from 4 different locations around diff.zip