Since you've already put some WD40 on it and it got better that tells you it's the belt which is either worn out and needs replacing or is loose. Tighten the belt if it still makes noise replace it. You can tell when these belts are worn just take it off and turn a few inches of it over so you're looking at the "ribs" then bend the belt back if you see cracks in the ribs the belt is worn out and needs replacing even if it doesn't make any noise. If it was the pulley the noise wouldn't change when you used the WD40,pulleys make noise when the bearing are worn out and they are sealed inside the pulley. Good Luck
