I had heard the tapping on my 3.0si when idling next to a wall at the drive-thru (so I could hear it pretty well). It's not very evident otherwise. And it only happened once or twice back in the winter/spring. Not happening in the warm months so far. Had never brought it in for the issue.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr