2008 Ford F-150 hissing noise

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.