I had Adaptive Drive on my 2008 X5 4.8i...and I had just what you are saying. My 2008 and 2009 ML63 AMGs (with Mercedes' version of AD) also had this kind of sound.

My 2007 E70 and now my 2011 E70; NO Adaptive Drive, and none of these sounds. Just from my experience, the Adaptive Drive might be the reason!