My cab seems to pressure up when the heater is turned on and taking in fresh air, it whistles very loudly out the drivers window. If I crack a window the whistle goes away, it also goes away if I put the heater on recirc. Could my cab vents be stuck closed? Any ideas as to what the problem is? Thanks

 

Yep, that's what they do.

 

your door could be tweaked also, amazing how easy you can flex the top corner of the door and get it to align moulding to cab, or in your case, it seperrates and then you hear the noise. look for signs of the seal not touching the cab, and flex the door do see if you can get them together and doing their job.

 

I would check the cab vents. I have 2 good sized ones behind the rear seat. As long as yours are able to open you should never need to crack the window to stop the whistle.