If the noise stopped when you put the soap on it ,I would tighten the belt a bit, if you can move the belt more than its thickness its loose but make sure you have some movement, if the noise is still there try the soap again.
Why soap works is a bit of a mystery to me, as it is made up of part oil/fat and part acid, maybe it cleans the belt and pulleys and squealing is a result of the belt slipping on the pulleys, but hold it there long enough on both sides and the squeal goes away, well it has for me anyway.