Hi everyone, two years ago I bought an 86 Stingray 18ft bowrider with the 170 horse 3.7 mercruiser. When I first took it out on the water it didn't take long for me to tell that it needed attention. Leaked oil like crazy was running hot and not running well, had water coming in. Long story short this past winter I replaced the Oil pan gasket as well as the rear main seal and fuel pump. I also did a new gimble bearing, bellows, u-joints, shift cable, raw water pump, and completely rebuilt the carb and even fixed the infamous cam shaft seals on the engine water pump (closed cooling system). This last summer it ran great! No oil leaks, no water leaks, no over heating, tons of power, only one thing..... with a load on the engine and above 1900 rpm there is what seems like some type of knock or tapping noise. It is loud enough for me to hear while driving. The boat will not produce the noise out of the water, there needs to be a load on the engine. It is tough to determine where the noise is coming from but it is driving me nuts! When I did the oil pan I did not notice anything out of the ordinary. Each cylinder has about 150 psi according to my compression tester and other than this noise it runs great. I tried to mess with the timing to determine if it was detonation and although this changed how loud the noise was it did not fix the issue. If anyone has any insight into what could be going on that would be much appreciated. I have a video of the boat in the water making the noise and a video of the boat out of the water I can post on YouTube if people are interested. Thank You