I have been noticing on my 04 2500 quad cab a wind noise /
( whistling ) at about 70 mph, I thought it might be the weatherstripping between the front doors. Mainly passenger side, but it sounds more like it was coming from the front of the truck. I run my a/c in the recirculate mode and adjusted it to the outside air mode and the sound went away, So I tried it again and again, sure enough thats were the whistling is coming from not the doors as I thought, It's going to the dealer on Thursday.

 

Someone else had the same problem as you and this was the final result:

Well, I finally did get it solved. After some more troubleshooting things seemed to point more and more to something within the HVAC system itself. My guess was that there was a slight leak at what DC calls the 'blend air door' in the air-handling part of the HVAC system (I believe there are two, controlling recirc and hot/cold air mix). I took the vehicle to a dealer and asked that they perform the electronic self-calibration routine required after these parts are replaced or serviced (you need a DC scan tool to do this - it runs the doors back and forth and resets the limit points). They performed the procedure (took all of 2 minutes) and it fixed the problem completely. Whew... would have been a real bear to fix if I (or they) had to tear the HVAC system apart. Love those electronic fixes...

Anyway, hope this helps someone else...

 

Here is the link to the full post:

https://www.dieseltruckresource.com/...threadid=39515