Now a question for you...how hard is it to remove the Idle Control Valve on a Z4 (M54 engine)? I want to take it out and clean it, as I heard on the Z4 forum I belong to that it also contributes to the hissing sound when slightly depressing the throttle. The intake boot they replaced lessened the sound, but it's still there and I'm OCD about shit.
#8 http://realoem.com/bmw/showparts.do?mod ... g=11&fg=40
I'm pretty sure they are nice and easy to replace. You might have to break off the hose clamps holding the tubes on, depending on the design, and replace them but other then that should be cut and dry. I'm going to warn you, it's almost rule of thumb to never remove an electrical component and clean it out. I have done it in the past and had poor results. The only part I ever remove and clean is the mass air flow sensor and I use this:
It's very possible you could have a small vacuum leak. Small enough to keep the check engine light off but big enough to make a noise or you could have a dirty throttle plate.
If your going to be in the philly area anytime soon I could always smoke your intake and check your fuel trims with a scanner (http://www.stealth316.com/misc/obdii_fuel_trim.pdf
). This is an absolute test in finding vacuum leaks but if not, try cleaning out the IAC but only with sensor safe cleaner. Let me know what you do, i'm really curious if it fixes your noise.
*edit* I'll run your noise problem through my research forums to see what hits, get back to you.