do the soap trick. that will tell you if it's the belt or the pulley. if the noise stays, it's obviously a bad or loose pulley.
then you buy a stethoscope with a rod on it and place it near each pulley while running (BE CAREFUL, DON'T LOSE A GOD DAMNED FINGER). You will know which pulley it is, because as soon as you get the scope on it, the noise will be horrifyingly loud.