2003 Merc 15hp 4strk engine knock at high RPM

[jmcconne]

Picked up a little used Merc 15hp outboard for my boat and took it out on its maiden voyage. Runs like a champ and much quiter then my 20hp 2 strk however at 95% and up power it has a knocking sound that increases with the engine rpm. I originally thought a rod knock from the sound but now Im wondering if its some sort of limiter. Any ideas?

any help or ideas appreciated

Thanks,
Jim

 

[jmcconne]

https://youtu.be/tZKUy-E5tHg

here's a video with the noise