I have also experienced a whistle upon acceleration since I have owned my car. I took it to the dealership several times while on warranty and they told me it was normal. They even took me out in another X-5 and it did the same thing. My car now has 171,000 miles on it and running strong, and it still whistles. Have had DISA valve replaced at about 95,000 miles for another problem and it still whistles so don't think that is what will fix it.

Hope this helps!