I don't know if this is the case, but if vacuum is not sufficient to hold door in position it might allow high force wind to overpower controls. You may need to remove glove box to maybe get visual view of what is happening.

 

hey hanky, checked vacuum at both vacuum motors to hvac door controls: 22 inches steady. this van has an air bag where there would ordinarily be a glove box. there is an access panel on top of dash so i can see that the controls are operating correctly but the air box is accessed by removing the entire dash. i'd like to eliminate everything else before doing that if i can, but happy to do it if it comes to that being the obvious source of noise. i'm going to leave the access panel open and have someone listen with a modified stethoscope to see if the noise is coming from there. it's hard to isolate the direction of noise in this uninsulated work van.