Originally Posted by X5only

Not the level, I meant draining old fluid and refilling with fresh dealer supplied fluid. It's pretty cheap. I would try that before anything else. I almost replaced/rebuilt my whining transfer case (as recommended by Indy and dealer tech) but decided to first drain and refill the fluid. The whining gradually subsided in a span of about 300 miles and then completely stopped. That was like 10k miles ago.