Anybody else get this sound?

 

I am in the Glass business and I have to fight alot of air noises. I have found that the cabin pressure can become weak in places like the door seals after the doors start to sag a little. the whistling could be coming from a point like that. The way you could tell is by releasing the cabin pressure by rolling down one of the windows about 1 inch. and see if it quits whistling. Air leaks are sometimes the hardest to find. Hope this helps.

 

If its only there when you turn the ac on its probably caused by that and not an air leak. Do you hear it right in the cab or under the hood?