The dealer and I determined this was coming from the passenger side rear view mirror, and is a very noticeable noise when driving with that window down above 50 mph. They claim that this is a design flaw and it is present in all X3's to some degree. They also explained that they determined it to come from the seam/gap on the top of the mirror near where it dips down towards the car. I had an X3 loaner and did not have any of the same issue. They offered to either caulk or tape the gap to get rid of the noise- and that they sent a note to the BMW field engineer. I really don't want to alter my brandnew car and add caulking.
I don't believe that this is a design flaw and that nothing can be done. They will not replace the mirror housing because they said when they drove two other in stock X3s they all had the issue, hence no reason to replace the mirror.
Is there anything I can do? Any suggestions from someone who might have dealt with a similar issue?