The whistle is Bov, if you have a greddy my doest under partial throttle just doing what it is supposed to.

 

Pressure release valve or more commonly known as a "Blow Off Valve", releases turbo pressure when the throttle plate is closed. The turbo is still spinning and still creating pressure. The forced air will hit the throttle plate and return where it came from. When a BOV reads vacuum from the manifold, it either opens a valve, or softens the valve. Which lets the pressure escape from it's opening. In order to work correctly the air must go back into the intake before your turbo because the Air Flow Meter has accounted for it. If not you will have a temporary rich condition which will upset your idle slightly. Proper tuning can get around this hassle. Most choose to vent to the air, as they love the sweet sound the air makes when it runs to the atmosphere. If the pressure release valve (as some call it) contains a horn or small holes/vents it will cause the air to make a louder/higher pitched sound depending on what is used.