Diff noise

Under the car this morning changing the dog bone mount, when I rotated the drivers side front wheel I could hear a low squealing noise coming from the auto-trans diff.

It sounded like a brakepad rubbing on the rotor but it's definitely coming from the diff, I had my ear right next to it.

Would a transmission fluid change sort this or do I need to dig deeper? Just looked at a manual and there's a bajillion parts in there.