The TDI I had does not have a BOV as there would be no way for it to work as there's no throttle in the intake. Thus when you let up on the throttle, it cuts the fuel, that's all. So on my TDI, was a VNT (Variable Nozzle Turbo). It had a vacuum pod on it that changed the nozzle size. That vacuum pod was hooked up to a valve (which goes bad on many of them). All it does is regulate the boost (which is around 20 psi on the model I had). The whistle was the sound of the turbo.