Does it start snapping, cracking, and popping? Does it appear as though the air flow isn't fully where it appears to be?
If so then check out this thread
, then get your truck on it to the dealer.
I had my A/C recirculation and temp actuator replaced due to failure. My 3 yrs/ 36K had expired (even though I only have 26K miles). They covered it under warranty, which leads me to believe that this is a known problem.
Good luck and keep us updated