Greetings!

I have a 19' runabout with a 1997 Mercury 150 hp outboard on the back
of it. I've had the boat for a few years and have never had an issue
with it until last week. When idling, in reverse or neutral the engine
sounds fine. However, when I put it in forward there is a knocking
sound coming from the engine. It seems to correspond with the engine
revolutions and speeds up as the rpms increase.

I know very little about engines. What I am looking for is if anyone
has any thoughts on what this may be or what I can do to narrow it
down. Could this be a sign of an engine on its last legs?

Thanks!
Ryan