After a 100+ km drive to the Christmas Family meeting, I am 100% sure the problem isnt fixed with the new rack. The vibration is still present, although it seems a little less prominent.
I discussed and demonstrated the issue, and while we were talking inside my BMW juice shaker, I shifted into park and noticed there was no vibration anymore. I then shifted into D again, and there it was again. When driving at a constant speed the vibration is more prominent, but we all agreed the frequency seems the same, regardless of the speed.
Warm engine or cold, the symptoms stay the same. As soon as the car is in D (as opposed to P or N), the vibration comes alive.
I am no expert on 3-series internals, but i think the engineers should be looking at things like the gearbox and drivechain. After this discovery, we drove again on the freeway at a constant speed of 100km/h. We started in 8th gear, and then shifted back one gear at a time. The frequency of the vibration seemed to be constant.
Which part rotates a the same frequency regardless of the selected gear, given revs and speed?
I will report these findings back to BMW, demonstrate it, and wish them good luck with it.