Hissing sound while at highway speed

[Guanfy]

So I noticed during a test drive last night that when in 5th gear at highway speed I have a rather loud hissing noise coming from what sounds like behind the center dash. It is linked to the accelerator. when I let off the gas and coast it goes away completely. I don't believe it to be a vacuum leak as I only hear it in 5th gear and all of my vacuum connections are either capped or connected. I don't think it's exhaust as the manifold gaskets are new and I just replaced the gasket it at the head of the cat. When the truck is at lower speeds or idle it does not make this noise. Anyone have some thoughts about what this might be?