Its hard to tell with the backround noise but I really only noticed it when I would first start it. The easiest way to tell is to disconnect the power from the solenoid. If you don't hear a different then your solenoid isn't working or you have a hole or leak in the hose to the flapper valve. Personally I didn't like the sound it made when there was a leak in the vacumn hose.

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E83 - Aero PKG Mods installed: Homelink mirror. CIS-IBUS video module, AUX, Rr Dr Release, stock wheels, Remote start, CarPC (Atom) & mud flaps - NAV - MKIV - NA Map DVD - 2011
E46 - 18.5 wheels/tires (summer), 17 stock (winter), Homelink, NAV software upgrade, BMW video module, Radio to V53, AUX, USB IBUS interface,BMW alarm install, CarPC(Epia), remote starter, tank battery & angel eyes. NAV - MKIII - NA Map CD - High Version 2008.1