Center diff/transfer case
I had a similar issue with my '01 330xi, starting around 130K miles. Very subtle drivetrain vibration, sometimes visible on the gearshift lever. Got gradually more noticeable as mileage ran up to 145K. However, it was noticeable only at very light throttle in 4th/5th at 45-65 on level ground. My indy eventually traced it to the transfer case (center diff). I never had it replaced and traded it in. The car was only worth $9500, and I would have been in around $4K in parts for the transfer case and clutch (if you're gonna drop the tranny might as well at that mileage).