What is the dash tapping noise when I turn my vehicle off or a/c on/off?

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I have had this problem for a while. The dealer told me it was a blow out door or something to that effect. I can be driving and all of a sudden I hear this tapping noise. It occurs when I stop the engine sometimes and when I turn the a/c on. It sounds like it's right behind the dash. I still hear the tapping noise and in addition, it sounds like some paper or leaves or something is stuck and just flapping behind there. What could this be? Thanks!