Where is the "flapper" for recurculate vs outside air?

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I have a 2001 quadcab that has an annoying whistle at highway speeds... I originally thought it was a loose antennae, but it wasn't. However, I have diagnosed the problem as the "flapper" or whatever you call it that seals the a/c system when running in "max" mode (no fresh air). Apparently the flap isn't sealing good and when enought air pressure builds up over the hood (at highway speed) it pushes the flapper barely open (just enough to whistle). I could take to the dealer, but if it's easy to get to I'd rather do it myself.

Thanks-dm

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2001 SLT dominantly valved ETH quadcab 4x4 dually
icq61308432

 

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It should be right behind the glove box.

 

[Archive]

Josparkz is right. If you just open the glove box you can see the flapper behind it by looking through the crack over the back of it.