I promise that I will eventually get over the fact that I bought an '07 Z4 3.0i recently, and will stop trying to analyze every single sound the car makes. I noticed a distinct hissing sound (like the sound a very thin water jet) whenever I step on the throttle. Even a slight acceleration causes this. The sound changes every time my gears shift (I have automatic gears). I do not hear the sound when I am not throttling.