I can’t say how those scratches got there, but I don’t think that such would the source of a knocking or banging sound.
I would do as you said and first pull each valve cover off one at a time a see if that makes any changes.
If no changes take place I would get a length of 3/4” heater hose, take all the belts off the motor and listen on the side of the balancer for the knocking sound.
In fact I would be inclined to do this first even before pulling off the valve covers because if you hear anything from down there, then it anit from the valvetrain and you really do not want to run the motor anymore for troubleshooting.
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I do stuff for reasons.
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