well ive got a 97 5.2l and ive just changed the belt and yet i still hear a whine. ive taken it to a few of my mechanicly inclined friends and they said it sounds like the idler pulley. the sound whines quicker when i on the gas and when i left off it slows down and at idle it does it every few seconds which is why i also think its one of the pulleys. in input would be great.

 

Take the belt off, spin your pulleys, see if any of them don't feel quite right. Check the belt, see if there is anything in any of the grooves. Clean the pulleys.