My boat is making funny sounds

It sounds like the sond is coming from the prop. I have a 1990 prostar 190. There is a knocking sound down by the prop area when I put my boat into gear. It is extreamlly bad if I put it into Reverse. I never use to hear this sound. I have looked at the prop there does not seem to be anything wrong with it except a little ding in all 3 blades from a stump. The mark is probally no bigger than a pencil led tip. So I don't think that would be it. Can anyone help me. My belt seems to be squicking quite a bit to but only when it is in Neutral or very very slow. Thnks for the help.