Intermittent scraping noise coming from engine

[SkiBoy]

Sometimes happens in gear, sometimes at idle. Not every time I operate the boat.
Boat is running fine not over heating.
It's a short kkkkch and then nothing. I rev the engine and all is good, or just keep it at idle speed.

Some days no sound at all.
Any ideas?

I have checked oil level and trans fluid, all good.

Thanks