Hello, I have a 2004 F250 6.0 Diesel. I have a whistling noise coming from the firewall area. You can only here the whistle when you are driving and the rpms are up. It will not whistle at idle. At the same time I have put a new radiator cap on and thought that might have been the problem. It leaks antifreeze out of the cap under pressure. I have put a egr cooler delete kit on because of the cooler problem. Can anyone help me with the whistling noise? Thank you...

 

If the sound is high pitched it might just be turbo whine.

 

I Have an 05 6.0l powerstroke, and the whistle u are hearing is the turbo like horchataking was saying. I know i can hear mine very nicely around 10 pounds of boost after that the whistle dies down. Thats the sound of power and enjoy it cuz thats what lets u know u have power under the hood.