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HI I have a Mercruiser 470 engine that has just developed a knocking noise while i was out on the lake it started to get louder and louder all of a sudden. I was hoping i could get some ideas im really hoping its not a rod or piston.Hear is what i have done so far I started it up on muffs in my drive way and ran the engine both in gear and at idle and knock was stilll there .I took each plug wire off one bye one with the engine runnig to see if the knock would stop it still continued to knock no matter what plug wire i disconnected . So my question is ? if the engine still knocked after each plug wire was disconnected dose that rule out the thrown Rod and Piston slap, bad bearrings .Is that a for sure way to rule it out ? its hard to tell where its coming from it sounds like either the front of the engine bye the harmonic balancer or the rear of the engine. It sounds like a bad bearing grinding/rubbing I have replaced the gimbal bearing last year on my routine maintaince if I let the engine idle it sounds like a dryer machine with a bad belt lol please help !!! summers here and now this lol