Eisenmann Exhaust! I have a custom M5 Eisenmann exhaust on my 540. They are more expensive then other, but it's the best sounding exhaust you will ever hear (IMO)

Check them out:
http://www.eisenhaus.com/ehprice.php...=67&Submit.y=8

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Hers: 2005 X5 4.4

Mine: 1997 540
(way to many mods to list)