My previous experience with PO'wned tcases that were built has not been great, so I'm gonna go ahead and say pull it and have a good look. It could be something as simple as a misplaced shim or thrust bearing, but the only way to find out for sure is to disassemble, clean and inspect. Oh, and if a gear was touching the inside of the case AT ALL, aluminum will be in the oil. You just cant see it because the oil is blackened by other contaminants, including the case material. That's my theory anyway. I just went through a  similar scenario with my tcase and it was buggered pretty good, but those expensive gears were salvageable, just.