hissing when give her gas

I'm looking for help diagnosing a problem we are having with our 2004 PSD I'm fairly sure it a turbo but thought I'd pick yawl's brains and see what you think. We left last Friday for a short camping trip and about 10 minutes into the trip I started thinking that the old girl was running a bit...

When you give it gas - there is this low hissing sound and it tends to feel like it's losing a little horsepower - anybody have any idea what that could be? Maybe a vacuum leak or something with the turbo? Thanks in advance for any thoughts...