i recall reading that every so many car start/stop cycles the vents inside will reposition themselves from full open to close. given they are electric i think it's just to ensure they are in the right positions.
conceivably they could get into the wrong position vs the controls itself.
long way of saying...absolutely normal. the noise however should be very faint. when u say loud like a garage door opener...i may guess the servo motors maybe ready to pass up to the servo motor grave yard. no clue on this..just my hypothesis...
mine were always very quiet...you really had to listen for them by the air vents. in fact when you adjusted the vents you could hear them move. but very faint.