Hi,
3 weeks ago i did recaived my new Z4. From 1st day i could have heard some rattle nois from back side of driver of interior
It seems to sound pretty similar as you described.
After 2 weeks of investigation i figure out
two source of rattle sounds:
1. First one is plastic cover of the back left roof top mechanism. Whole plastic was making strange noise on not-slight-road. The plastic is under number 5 on the image. My dealer took it off and asked me to ride a little bit with car - and you know what - sound gone
Dealer will replace this plastic with new one on guarantee.
2. Second sound i can hear is exacly what you describe - rattle from the back of interior. When you drive and hear that noise, try with your hand (or with you passanger help) to push your rear window up - sound should stop. You can also open a roof a little bit and try to push your rear window down and up - you should hear that sound. Im going with this to my dealer next week. I will write you does he help me. There is propably problem with window fixing somewhere there.
Let me know if i could help!
Regards, Simon