I just got my new turbos from BMWna also. I have wg rattle during acceleration at about 25-30ish % throttle. Wot no rattle and no rattle at startup.
To be honest the car sounds like shit. I'm going to bring the car back in this week, my SA asked me to drive it for a week to see if things change, but I have a feeling BMW is going to tell me this is "normal" (and we all know it isn't)!