I have a 2005 X5 4.8is in San Diego, too. I can hear the whistling noise occasionally. When I do hear it, it does bug me. Have not explored getting it fixed as I thought it was a normal X5 noise. Let me know what action you take and if BMW SD treats you well. Thanks!

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2005 X5 4.8is/ Imola Red/ Cream
2002 ///M Roadster/ Alpine White/ Imola Red (SOLD!)
2008 ///M5 ordered and on it's way!!