I installed an aFe air intake and my 2000 328Ci makes a whistling noise when "coating" (low-mid vaccumm in the intake manifold). If I accelerate or decelerate the whistle goes away .... the noise seems to come from under the car. If the original air filt/box is intalled the whistle goes away. If I remove the original air filter/box from the mass flow sensor and run the car with out an air filter the whistle is back. Did the same thing with my son's 2001 330Ci and his does NOT make the whistling noise. All this makes me believe that there is a vent valve in the air intake manifold or a small air leak that once the vacuum pressure in the manifold increase it closes and the noise is gone. This thing is making me crazy and no one has been able to tell me what is wrong. Took the car to the BMW dealer, thye checked and smoke tested everything and all they could say is ...we put the original air-box/filter and the noise is gone.