I have an 01' 3.0 (purchased 2 weeks ago) and I have the same problem.
I can notice the whine/whistle, just barely as the X sits idle, but until you hit 1800RPM the whistle is minimal. My X is loudest from 1800-2200RPM, once you get above 2200RPM I think the engine is loud enough to cover up the whistling.
(this is my theory)
Since the X5 has variable intake system the intake is opening just enough for the whistle to occur, once you reach a higher RPM the intake is opened up wider making the whistle disappear. If you are on the Highway and give it enough gas to land in the range of 1800-2200RPM you will hear it and that is why....
The general consensus i have found is that it is the intake adjuster...
Pic Here:
http://www.xoutpost.com/attachments/x...g?d=1119978382
Diagram Here:
http://www.xoutpost.com/attachments/x...g?d=1120769730
I have found several people on the board that have replaced this part and fixed the whistling. Also, one guy said you could even put your hand over the intake adjuster and it was quiet( the noise that you can just barely hear at idle).
Link to "Whistling Noise Fixed" Post:
http://www.xoutpost.com/x5-e53-forum/...stem+whistling
I have haven't had time to put much time into fixing this but it is driving me crazy, ecspecially with nice weather outside and wanting drive around with the windows down and having to hear the awful whistling sound....
Hope this helps, just though I would through some more info out there for you guys.