Update... Just came back from the dealer and the rattle is definitely gone! They performed this service bulletin titled "Noise from Rear Trunk Lid"

http://www.bmwtis.com/tsb/bulletins/...p/B513710g.htm

And here's the corresponding PDF pictures on what they do...

Attached Images

BMW X5 Tailgate B513710_Attachment.pdf (376.1 KB, 3773 views)

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2004 Porsche 911 C4S X51, Seal Grey / Black
2006 Porsche Boxster S, Guards Red / Sand Beige
2008 BMW X5 4.8i, Black Sapphire / Saddle Brown
2001 Audi A4 1.8tq, Laser Red / Black, APR Stage 2, H&R Coilovers
2002 Suzuki GSX600 Katana, Yellow