1996 200hp making noise when in gear

I just helped a friend of mine mount a 1996 200 hp motor over the weekend. Everything on the motor runs smoothly under idle, but when we placed the motor into forward or reverse there is a clicking noise that comes from the shaft. This noise is realy only present under an idle speed, when we rev to 1500 rpm it seems to disappear. We only came across this problem when the prop was on, and when we took the prop off it was fine. Anyone with feedback please post.

[boats.net]

Probably prop chatter, if prop is in good shape torque prop nut and adjust idle speed to specs. May have to replace wavewashers and spacer on top of water pump impeller under water pump housing.