RE: My truck can whistle
 

It seems like it only does it when it gets hot outside, because before it wouldnt do it in the mornings when it was cool out but later in the day it would and now that the days are getting cooler, I don't hear it as often. So now that will make it a little harder to diagnose now that I can't listen to it anymore. Also one day I was messing with the A/C control knob (the one with "Max A/C" etc) and I was slowly turning through each thing and when I got to the last few, I would get a whine that would go in addition to the whistle, except the whine wouldn't stop until I turned the knob, then it would stop and then go again, and after I got it to start whining it would do it on all the other positions except max a/c (I think, this was about a week and a half ago I cant remember and I havent been able to do it again).