It's your recirculation door opening and closing.  They tend to get a bit
noisy after time.  My 88' does it when the key is first turned to ON, and
then when the vehicle is shut off.  Sounds like a cow in
labor.....................

I wouldn't worry about it, the electric motors just get a tad noisy over
time.

Doc

The recirculation door motor is bad, There are plastic gear inside the motor
and they get stripped. When you shut down the truck, the motor is trying to
find home position.
The motor is a royal pain to replace, the service manual will tell you to pull
back the dash to replace motor.
I have gotten away without doing that by using a 1/4 inch air rachet, a mini
extension and a 5.5mm flex socket, and looking for the upper screw by feel.