If it was a coil or plug, regardless, wouldn't this ecu and knock detector show a problem before ours ears can hear it?

Never heard a fuel injector that loud.

Can pull the plugs on that side and inspect each one and would tell you if there was a fuel or spark issue.
alot can be told about a engine by the condition and color of the plugs.

A vacuum gauge can tell the condition of a engine, if vacuum is good and steady needle on gauge.