'01 supercrew w/ 5.4

Trying to track down the culprit surrounding wind noise that I hear coming from the plenum area behind the dash when my HVAC controls are set to Max AC and driving at speeds above 40 MPH. In addition, the faster I drive, the louder the wind noise becomes. When in town at slower speeds or parked, the wind noise is non-existent. When I change the controls to (regular) AC at higher speeds, the noise disappears and all is well.

I've searched with limited success (additional difficulty without the proper keywords) with the only lead coming from this one post here on FTE.

In looking at HVAC diagrams, I'm fairly confident that the issue is either A) the seal on the "Air Inlet Duct Door" is bad and failing to keep outside air out when the door is closed or B) vacuum holding the door closed is weak when in Max AC (recirculating) mode and thus air is getting around the seal. My gut feeling is the seal more than vacuum because I hear the clunk when I change to from Max AC to AC and hear/feel the difference in air pressure coming from the vents.

Regardless, I wanted to ask the community here in case someone else has or is experiencing this and can chime in. I think I have to pull the dash and open the plenum to change the door, but am not 100% on this.

Thanks