Hi, I have a Johnson 70 think it"s an 1989. I was coming back in after a trip and a knocking noise started coming from the engine. I layed off the throttle and the knocking stoped for a sec and started again then kept getting worse to the point of where there is no drive to the prop. When I select forward or reverse now there is no continuous drive just the odd crunch. Has anyone got any ideas of what this might be and also how to go about checking what"s wrong? Thanks in advance