My new to me boat has started knocking. I have only had it 4 weeks and not even made a payment yet!!! The hour meter says it has 220 hours! It all started when my fiancee couldn't start it when we were launching it. Once it was in the water and away from the dock it idled fine. It seemed sluggish getting on plane and didn't seem to go very fast. maxed out at 40mph at 5000rpm. We were very light and had it going faster before with the same loadout,got it upto 44mph at 5200rpm. Well, she was driving and I was trying to enjoy the boat but I was noticing it seemed to act funny. After we went about 2 miles she stopped and I took over the helm. I slowly put the throttle to it and it started ticking very loudly. I stopped, put it neutral and lifted the engine cover. Just about that time it started knocking. I dropped the cover and headed for the dock. I tried to keep it slow but I didn't want to get stuck in the middle of the lake. I kept it down to what the speedo said was 25mph and the tach was saying 3200rpm. It seemed to be loosing power as time went on. We kept it running until we were sure we had the boat on the trailer properly then shut it down. I pulled the oil dipstick and didn't see any metal. The engine oil looked fresh and clean. All the other fluid levels looked good except for the drive resevoir. It looked low again. It seems to be leaking out the top of the cap like it is pressurizing and flowing out the vent hole. I've rebuilt vehicle engines but is there anything special since this is a marine engine?