73 115hp johnson OB all 4 pistons have 118to123 compressions lower unit has no metal on magnents sound is only there when boat is in water and gos away when at full throttle i am anewbie at this so i have read alot of stuff on knocking sounds but still lost thank alot for the help
several things will help, your location .
is this outboard new to you?
did it start doing this recently?
did it run ok before?
Does it knock in neutral or just in gear?
Have you done any work on it recently.