Pretty much every BMW I have had the pleasure to drive or ride in had some rattles. If having zero rattles is important to you, take a look at a Honda or Acura, Ive owned one of each and after 5 years they both had zero rattles. Unfortunately they also have zero soul and had zero 'fun to drive' ness. My feeling is BMW is going for very lightweight materials and in order to maintain strength they are rather stiff. Combine that with a stuff suspension and you are going to get rattles where they meet.
If you can find the pieces that are the most troublesome, you can lessen the problem with felt tape. I had a Toyota pickup that was a rattletrap within a few months of buying and I chased all the bad ones down this way.