2008 Ford F150 V8 Two Wheel Drive Automatic 20,000 miles
I have a 2008 f-150, when I purchased there was no noise or any problems, at about 4,000 miles it started to make a loud hissing noise almost like a vaccuum leak, the sound only occurs when you hold the throttle steady and at about 60 mph it is very loud, when you step on the gas it goes away and when you let off the gas it goes away, as I said it is only at a steady speed, the dealer says it is not right but an engineer couldnt figure it out and stated it is a characteristic of the vehicle. However when you test drive the same vehicle there is no noise. The dealer has rplaced the intake manifold, and catalytic converter, but will not do any more for this noise.
I had a similar noise that the dealer had a lot of difficulty tracking down. They replaced the throttle body after a couple of months of trying and that seemed to fix the problem. Looking at it there was nothing obviously wrong but the truck works great now. Hope this helps.