I'll chime in here. I just got my 2012 X5 35d and I am very happy so far. The "rattle" you hear off idle is actually timing rattle and pretty common on most HPCR diesels. I own Left Coast Diesel, and we do everything from oil changes to competition builds on diesel pickups. My other truck is a 675rwhp 2004 Ram diesel and it does this pretty badly with the larger injectors and programming I have in it.. anyway, as timing is increased, especially at low rpm/high load condition (like off idle or tip-in at cruise) you will get some rattle as the rpms increase to match the timing in the map.
Hope this helps some.