EDIT: traded in my 3.0i for 4.8is.
Problem fixed 
Below is the original post I started regarding a whistling sound my X5 makes when driving at low rpm's and pressing on the accelerator.
It's similar to the whine of a supercharger, but annoying.
http://www.xoutpost.com/x5-e53-forum/...stling-x5.html
Based on the feedback from fellow members, the dealer tried the following...
1)
Replaced DISA valve - no change, replaced w/original part.
2)
Replaced Idle Air Actuator - no change, replaced w/ original part
3)
Replaced the throttle body - minimal change if any, kept new throttle body
4)
Checked air filter box for any leaks - none found
Service manager claims the whistling is due to the vehicles age.
2003 X5 w/ 48K
He and the tech drove a newer X5 and whistling was not there. Then they drove a '03 w/similar mileage and whistling was detected.
Three of my co-workers also drive X5's. Two do not have the whistle and one does.
Does anyone have another idea?