Super loud diff
 

Buddies car.

Axles are disconnected, tried 2 different drive shafts.

Still makes a nice howl.

Toasted diff?

Nothing?

It's kind of hard to tell anything from the video.
Your test will leave the axle stubs (if na8) rattling around and could explain the sound. Also the sounds that the diff will make unloaded don't say a whole lot about its state of affairs.
How does it sound driving around under load?
I picked up a diff and used it for 5k miles and it always had a ton of gear whine.
I eventually pulled it out and found that the backlash was 0.040 instead of 0.004. After setting it up correctly the whine has been substantially reduced.

Ignore the rattling. Try and hear the whine. It sounds the same loaded and unloaded.

I'll have him pull it so we can check backlash.

It sounds bad, but why have axles unhooked? I am not sure I would pass judgment until diff is properly loaded. Also, I was told never to run a diff unless it was under load. True?

We were eliminating variables. It makes the noise when driving too.

The noise is not related to the load.

I would say you have a problem. Has there been a gear change?

Nope torsen was sold to him as is.

Hrmm yeah the backlash noise was only under load and after about 15 minutes climbing the mountain for me to get home it felt like it was piercing my soul. Everyday.. haha
That sounded more like a hum in the video? Does it sync with the pinion or the pumpkin?
Hopefully its a bearing!

It just sounds like super loud gear whine.

Damn the unknowns.

If if it were me and I already had the axles out, I would remove the chunk and do a gear inspection. It could be simply set up wrong.

The car is already back together. I'll have him pull it so we can do that.

Check the drain plug magnet for shavings? Any bits large enough to read the part number off of?

Or install new bearings, set the preload and back lash, inspect the diff, and change the fluid? I swear most Miata owners spend more time on diag than basic maintenance.

Yep it needs a rebuild. I had a similar sound in my stock 3.9 lsd and the bearings were toast. This sounds like backlash tho, which may be caused by worn bearings. So, it needs to be opened up one way or another.

I just spit coffee all over my key board. :)

Might be the worst place to ask, but could not think of another way unless I started a new thread. How much do a 4.1 vs a 3.9 torsen diffs go for respectively? Think there would be chance to trade a 4.1 for a 3.9?

I paid 600 for my 3.9 torsen. And it was broken. I got fucked.

I will try my best not to get fucked. Although, sometimes after a few drinks and a quick search online, it is out of my control.