I have a 2004 Z4 2.5 62K, for 2 years now, and have always had a rattle at low RPMs to about 2000 RPMs when it would disappear. I always thought it was a VANOS problem until I happened upon a youtube video "DISA_Rebuild.MOV" and a few others grouped with it ( BMW calls the DISA a Adjuster unit). I notice the same play in the valve plate that closes at low RPMs. The shaft that come up in the plate which is moved by the vacuum advance was striped so when it was supposed to be closed it was flapping and making a rattle noise. What I did to repair it was Clean It Well, removed all grease on flap with simple green and then alcohol. I found where the flap would sit and drilled a 3/32 hole in the base and applied a small amount of Super Glue in to the hole, IMPORTANT I held the unit so the flap was down to make sure the glue did not go in the base to stop the unit from working. Yeah I wish I would have taken pictures too. But, the rattle is GONE and the car has more power at low RPMs.
Watch the video and it will help you understand what I did. I did not take it apart just did whats above. Oh and saved $200.