I have the same. It's behind the sat nav, I actually think its the hard plastic cover over the speaker.
I spoke to bmw Melbourne about it, as usual they said they test drove the car and couldn't find it.
I'll be going for the test drive next time as its getting really annoying, especially when the car is cold with rough idle.
Good luck with your rattle!
2011 E90 M3
R33 GTR Skyline Track/Fun car
Landcruiser LS2 powered