If you're able to read and understand german (which I somewhat assume, dear dutchie
), you might wanna take a look at this thread in the german forum:
http://www.1erforum.de/der-neue-1er-...-h-117845.html
It describes exactly the problem, some people have it, many don't. They've come up with several (not working) ideas and recently with something more solid. At least 1 car is afaik "under investigation" by BMW currently. Seems to be an airflow-problem in the lower front area causing chassis-vibrations.