Intermittent Whistling noise

[2011BlueFlame 2]

Hey! This will be my first post but I have been a member for a few months while I was looking for a truck. 

 

With that said, I did find a 2011 F-150, 5.0L, long box and I have noticed an intermittent high pitched whistling noise, but it only occurs while at highway speeds. It sounds like it is coming from the dash area and it noticeable once above 60mph. It is not constant and there is no discernable pattern.

 

I have done some research but have not found the cause of it yet. Has anyone experienced this before? I have a video but have not uploaded it yet. I figured I would reach out first and see if anyone has experienced this before.

Edited November 22 by 2011BlueFlame

[Going_Going_Gone 29]

Off the wall comment, does the whistle change in tone as you accelerate or decelerate...or does the airflow switch from dash vents to defrost vents w/o you doing anything?  If so, it may be a vacuum leak to the blend door or thereabouts.