Groaning Under Dash

[BIGPROFITT]

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

[formula1]

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.

[BIGPROFITT]

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