It sounds like wailing a block of wood with a sledge once. Honestly maybe something floating in the water both times?Wow what a great job and so glad the pix are working again so we can see the finished product. About the noise I wonder if it’s the exhaust flappers; when I changed my exhaust from the one piece batwings to the 2 piece conventional exhaust the flappers did make a bit more noise at times which sounded like an engine knock just about drove me nuts. Finally found it with a mechanics stethoscope on the Y pipe…