Odd sizzling sound?

Now this is going to sound weird. For about a month maybe I have been hearing this sound while im on the highway. I have an Xcab so I can lean the seat back to try to find where it's comming from. It sounds like it's from the middle of the truck, maybe slightly to the driverside. But here is the deal. It sounds like a sizzling sound and only happens when I go over 50mpg and cruising. If I accelerate, it goes away and when I dont use the throttle at all it's gone too.

I cant figure out for the life of me what it is! I thought it was just something flapping in the wind like some tape or something, but it's deffinatly related to the throttle, and only at cruise... in other words just barly giving gas at all!


Any ideas????