Mercury repair questions? Ask a mechanic for answers ASAP
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When do you hear the noise the most.
Hot or Cold?
Idle or under load?
If you are in neutral and rev the engine do you hear the noise more?
A knock that is always present and isn't affected by load or temperature is typically a valve train noise, which you've inspected for, or a broken piston/skirt. Rarely a carbon ridge the top of the cylinder can make a noise. I recommend continuing with your current diagnostic path and cancelling cylinders by unplugging injectors one by one while it's running. If the noise goes away it's probably a piston or wrist pin problem.
If not and the noise is coming from the timing chain area pull the timing cover and inspect the timing chain guides. They are plastic and can wear causing chain slack and noise. This is typically a loud tap from the front of the engine.
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