1999 Dodge Ram Hissing Sound from Engine

I get a really loud hissing sound when the truck is under load only and accelerate between 2000-2500 rpm. Sounds like air being forced/sucked in at a high rate. I've changed all the gaskets, found the belly pan manifold gasket almost off and pan full of oil. Put it back together and still the same noise. Still going through alot of gas. I've put ether on the engine looking for leaks and no change in rpms, but motor sometimes revs high under load as well. Thank you for the help.