Evinrude 150HP 1995 60degree motor, in idle or under load at low speed like 10 15 mph it sound perfect, at higher speed 20-30 mph , I can heard like a knocking noise , wish is not consistent, it sound different every time.. .. any idea what is the most common problem, I know I will need to rebuild, just to have a idea, a wrist pin, a rod, bearings.. if this append to you , just let me know..thanks