I think I know the answer to this but I am going to ask those of you who are more experienced than I. Today I noticed underway at high power settings I was getting a knock particularly bad when I turned sounded like it came from the back of the boat. We stopped and got a slip about 3 hours from home so I could have time to look it over. I thought at first something must be around the prop, but in the slip, transmission in neutral the shaft tirns really easily. I grabbed the shaft coupling and found I can bang the driveshaft easily on the Stern tube in any direction. Jssalem says go hire a diver. What do you say?