Engine Squealing Under Load

I have a nasty squealing sound coming from my engine that I can't figure out. It sounds similar to a loose belt, but not exactly the same. It only does it while driving with the engine under some load - accelerating, keeping speed up a hill. When idling or coasting down a hill it does not do it. I can tell that it is related to engine speed because it will get faster with the RPMs, not the speed of the vehicle. It won't do it while I am parked and under the hood trying to locate the sound, only while moving. Does anybody have any idea?

Thanks,

might it be a leak in a vaccum line? that gets more intense with RPMs.
Another thing is it may be your intake.. I ran a RAM air tube for a while and it sounded like a supercharger. high pitched whine. at speed. quiet at home. really hissed while I was hitting that passing gear. It didn't hurt anything, but got annoying, so I put the factory box back in and problem gone... MPG improved too.

I still would check for a loose belt. The need to be pretty tight not to squeal

I second the belt.