After many years and countless service calls on my 2007 MDX in an attempt to resolve the whistling noise that happens when the A/C is on and in recirculate mode I resolved it myself.
The dealer tried many different fixes ranging from resealing the windshield to replacing the blower unit. I highly doubt that they even replaced the blower as I had the same problem after the fact. At one point the tech must have run out of options and actually wired the re-circulation door shut.
Finally, I decided to tackle the issue myself and after taking the glove box door off and studying how the mechanism works I found a solution.
There is a cam that is actuated by a servo motor controlled by the recerc button. The shaft that is attached to the recerc door rides on this cam. After I looked at how the doors shut I noticed that in the closed position the door doesn't have any tension on it to keep it closed. At highway speed the pressure is greater than the inside cabin causing the air to leak past the recirc door.
I visited my local auto parts store and found a package of different springs for about five bucks. I chose a longer spring that has light tension. This part is where you have to use some MacGyver skills to fashion the spring end to fit around the door shaft without impeding its operation. Stretch the spring around the back and attach the other end to the bottom of the dash plate. This will take some trial and error to get the tension right. There cannot be too much tension or too little. Hope this helps.