I have that same sound. Been that way for 6000+ miles and who knows how long before I got it. Keep us updated as to what you find.

I got under the truck and poked around but didn't spend a ton of time on it. Was looking for something loose as it sounds like a loose metallic noise rather than chain noise to me. I found a loose heat shield but it was not the source. Need to get it on jack stands, get under it, and try to isolate the noise... I've just read weird things about how the X drive doesn't like being air born in gear. Any truth to that?

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)