I have a 99 powerstroke making a weird whisel/hissing air sound coming from top of motor by fire wall. Don't do it unless you are going steddy speed down the road gets boggy/slugish when it dose it. Almost sounds like bad boost leak out side the truck. Looking for info on what it might be its loud and gets old

 

Could be a loose clamp on the turbo that goes to over to your intercooler would be my guess.

 

Could be your exhaust back pressure valve...