HVAC Whistle - 2004 GMC Sierra

[zmaddox5306]

I have a 2004 GMC Sierra with automated HVAC controls. With the blower on 4 or 5 it has a loud whistle (sounds like a coaches whistle almost). It gets worse as I drive faster. Turning on recirculation stops the noise, regardless of speed of travel. This is no good in the winter becuase the defrost kicks off the recirculation resulting in a very annoying drive. I assume there is an air leak somewhere around the recirculation door/air intake. Where should I start looking to correct ths problem?

[zmaddox5306]

If I crack the window a little the whistling stops..,,

[zmaddox5306]

We'll I feel like an idiot. The passenger window was cracked ever so slightly causing the noise.