87 engine noise

Hello. I have an 87 cc riviera with a 351. Owned the last 4 years and when I put the boat in after the winter I took a spin and heard an engine noise. The noise only happens when I am at full speed on plane. The sound is sort of a small clank or denting a beer can sound. Happens once or twice about every minute. I also have a small engine hesitation at top speed and rpms. Gotta listen closer with my ears once I can have someone else drive the boat tomorrow. Any initial thoughts or suggestions are appreciated.