Wow, I've had this with my '06 550i. Same description: it occurs when turning sharply, driving slowly, often when parking. Happens only occasionally so I haven't talked to my CA. But it appears we all have a common problem.
Now lets see if we can find out how to reproduce it. Remember, as the Terminator said "if it bleeds we can kill it"!
2006 550i: Monaco Blue, Beige Nasca leather Ventilated Multi-Contour nee Comfort Seats (!), Sport, Step, Comfort Access, Nav, HUD.