This?
http://www.1addicts.com/forums/showthread.php?t=796343
I've run into this on a E90 335i with N55. The damn vacuum line to the exhaust valve vacuum actuator is corrugated. If there is a leak (vacuum actuator defective or line is disconnected from actuator without plugging) it creates a high pitched whistle/beep noise. Note this vacuum line runs through the interior on its way back to the left rear of the vehicle causing the noise inside of the car.