This rattling after a cold start issue was a problem for me as well and my local dealer could never figure out what it was. I think they had the car at least 3 times trying to diagnose it and could never come up with anything. Strange thing, after I got my Evolve tune, the noise went away and I've never heard it since. Could it be related to the delete of the cold start in the programming?
09 E90 M3 - Interlagos Blue / Black Novillo / Sycamore Anthracite / Eibach / HRE