Thanks for the replies, such a great bunch of fellow fisherman and boaters on this site! I do know that when it happens it is coming from the back lower end of the engine. I can't hear it coming from the drive unit as it is obviuosly under the water line so would have to pull it to check that. I had the drive off last year, and the gimble, drive shaft, and u-joint all seemed to be fine, but that was last year. That all being said, what is the best way to address the exhaust shutters? Not something I will do myself, so will seek professional help! Thanks again guys and good luck and safe travels out there.