Anyone know why Im hearing a squealing sound whenever I let off the gas?

Yea it stops whenever I hit the gas, and its only while Im in motion. Why would the belt or pullys make noise when I take off the load of giving it gas? and does anyone know how I can tell if its the pullys/belt. I know the throwout bearing is bad when I release the clutch it squeals, Im just waiting till I can find a spec 3 or dual friction centerforce cluch to replace mine with since id have to take out the tranny, could the sound when I release the as be from my throwout bearing also?