I have a very similar sound under light acceleration and only when it's cold ( sitting all day or night ). I am pretty sure my mystery sound is something in the transmission. And the reason I am assuming this is because I have already changed the filter/fluid and it didn't change. And I have herd a very similar sound from another bmw next to me at a light....

__________________
2006 X5 48is Black / Black - LED AEs - FCAB - GAS 1.2bar cap