In my 1994 Chevy 1500, the air recirculation door is electronically controled. When the ignition is turned on or off, the door slaps as though it is trying to close even though it is already closed.
sero is stripped,and thats what you are hearing,the motor is re-homing itself,and can not do that when gears are stripped.(but it will always try).
time for a recirc servo.