High-pitched squeal and no engine breaks

I need help identifying my problem guys. I have an ISX with VGT. I'm getting a high-pitched squeal when I coast down hill. The sound gets louder as the truck goes faster. It sounded even louder when I turn on the engine break. However the engine breaks do not really work anymore. They don't really slow the truck down at all anymore. The squeal sounds just like blowing air over a blade of grass. The Squeal is also present when accelerating at lower speeds. There is no squeal at all when driving on a level surface. It only happens when the turbo is either pushing or pulling. And now I noticed a sound that sounds similar to air escaping whenever I turn off the engine. It's coming from the rear part of the engine near the firewall. And another interesting fact I have lost about .5 mpg since all this began happening. Any help would be appreciated. Thanks.

 

Found the problem. Blown turbo mounting gasket. $80 for new gasket and bolts. Now just need to put it on. Could actually see the jagged edge of the gasket sticking out each side.