Yesterday while fishing with my buddy in his Statos with a 99 200 ficht we went into a cove and as we slowed down and started idleing over to the bank we were going to fish we felt a small clunk like we hit something. I looked at the depth finder immediately thinking he had hit bottom but we were in 15' of water. We continued on and fished the bank with no more noise or problems. But, when we started to leave the cove his engine had a terrible knocking sound. I had him take it out of gear to see if it was related to the lower end and the knock continued. We idled for about 10 more minutes and then he opened it up and ran about a mile back to the ramp. When we stopped at the no wake zone the knock was completely gone and did not reappear while we were loading the boat. We checked what we could see from the outside at the ramp and found no problem. I didn't have the heart to tell him but it sounded like a loose piston or something broke in the block to me. Any Ideas??