I suggest you look into the sunroof seal. My 2001 4.4 X5 had the same issue. The easiest way to check it is to open and close the sunroof cover while you hear the whistle. If the noise changes you have your answer.

The seal is very easy to replace. The old pulls off. The new is held in with tape that comes pre-applied.

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2007 Suzuki Vinson 500 4X4
2007 Toro 6.5hp
2006 BMW X5 4.4 ltr (his)
2001 BMW X5 4.4 ltr (hers)
1994 Honda XR650L
1984 Honda XR80

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Benjamin Franklin