[tcgoesch]

I have a 2012 535 gt x-drive that has just crossed 100,000 miles. On a recent trip after getting new tires, I heard a consistent whir in the driveline at all speeds. It is most noticeable on acceleration and quiets on decelerations. It changes frequency with vehicle speed, but is unaffected by engine rpm. There is no change in the sound during turns. I have never changed the oil in the transmission or the transfer case. I see no evidence of external oil leaks from either part. The sound appears to come from the area just below the center stack. The noise is consistent warm or cold. It makes me think of a chain that has worn. It does not sound like a worn universal joint of half-shaft joint. I have check my favorite BMW websites and do not see any references to this type of issue. Could this be a Guibo that is worn? Any suggestions or experiences? Thanks in advance.