[Dirty Harry]

I know this sounds implausible, but bear with me. I witnessed it myself.

I was chasing a clicking sound that I attributed to lifter/valvetrain noise. Nothing was amiss anywhere that I looked. The stethoscope was nonspecific in telling me the source of the noise. I took the fan belt off and ran the engine. No help. I removed the fuel pump and blocked off the opening, then ran the engine. No help.

Finally, I noticed that the big oval air cleaner base was rubbing on a spark plug wire to the point that the spark was arcing to ground there. The snapping noise was amplified through the aluminum base and it sounded like a metallic tapping.

An easy fix after a tough search process. You might look for something similar.