Groaning Under Dash

I have a 2002 Pontiac Montana. Every once in a while I will hear a groaning sound coming from inside the dash. I know it has something to do with the heating/ac because if I change the vent selection (where the air comes out) the sound will change or stop. When this happens I also hear something that sounds like a flapper closing. This all just started happening about a month ago. Does anyone have an idea?

Thanks

I don't have a definitive diagnosis, but maybe 2 possibilities.

Flappers are moved by vacuum motors or actuators. Basically a rod mechanism with a rubber diaphragm in the middle, housing on each side of the diaphragm connected to vacuum lines, and spring loaded to return to the center position.

Groaning could be a flapper vibrating in airflow because it hasn't made it to final position because 1) leaking vacuum lines or 2) leaking diaphragms in the actuators.

Thanks for the information. Where would you suggest I begin? I have done a lot with mechanic stuff, but nothing with duct type stuff.