1995 powerstroke weird turbo hiss
 

When I start the truck when it's cold in the morning it will idle for a couple seconds then the turbo or truck not sure what it is will start making a hissing noise, it won't go away for a while even after I get down the road. I let it warm up for 5 mins before I leave. It losses power when this hissing happens. When hit the throttle past half it goes away. Any clue?

Exhaust back pressure valve...