engine knock

I have an intermittent knock going on with my 7.4L Mercruiser Bravo engine. It only happens at idle, and when I move the throttle up at idle it speeds up with the RPMs, then goes away. Sometimes at idle, it will go away and come back. Engine temp, level, and oil pressure are fine. Boat starts...

Port engine (Mercruiser 454) purrs like a kitten when not in gear. But, when the transmission is engaged it knocks like crazy at idle speed. Seems fine at higher velocity, though. At first I thought maybe this darn E10 gas had fauled my carburator. I have a 1990 Sea Ray Sundancer 350, so have...