Nibblin, is it a Yammy prop or did you have something special put on, Yamaha have a special anti thrust spacer/bush. I had another brand prop on mine although it was 150 hp and when put into gear I would get prop slap until the revs picked up. This could be the source of your noise, if it isn't the prop then it's in your gearbox and I be back at the dealer. Remove your prop and see if the noise is still there.