Mechanical failure

When I returned to my car at the end of the work day, I could hear this unusual sound coming from inside.  It was a rapid staccato clicking sound.  (Kind of like the sound of a bomb in cartoons).  I emptied the glove box, not there; emptied the console, not there; went around to the passenger door looking under the car & seat, not there.  Shortly after I started the car and started to drive the sound stopped.  Each time I parked the car and then returned, I heard the same clicking sound.  I called my brother for advice, his thought was something electrical and perhaps I should be concerned about the battery going dead.  I am thinking, electrical fire!  At 6:45 I pulled into the car place down the street from me (it closes at 7).  I went in to try to describe the problem, sound demonstration included.  Of course, they were puzzled.  The young man who came out to the car immediately guessed it was the "blend door" on the heater but he could not understand why it made noise when the car was not running and even when the heat/air conditioner was shut off.  He did think it should eventually shut off and thus actually start in the morning.  I have to bring it back for repair--of course, they have no idea how long that will take or how involved it will be, but no less than 4 hours is the estimate.  I truly think these mechanics see me coming every time!!  (If it starts tomorrow, I am going to call for estimates at least I won't have to demonstrate the sound now that I have a diagnosis).