i think somewhere in the manual it says every 10-15(not sure of the exact number) times you start the car it does a diagnostic on venting system, opening and closing stuff like that. that maybe what you are hearing
it's louder when heat is on, but usually I just have the fan on and it does it.....I could have sworn it never did it before, only started in the last year....bmw said it's nothing...they can't hear it.......that doesn't suprise me.
it is a normal noise, when you turn the engine off the heating system resets its self so if there is a fault when the car is next used you can get ventilation if the system fails, it should do it every trip.
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