1993 Suburban Noise under passenger side dash

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.