Along the lines of plug wire routing........

Look for silly stuff first. By silly I mean simple and not related to the rotating assembly of the engine. You say it is an odd sound so that opens up possibilities.

Exhaust leak at the manifold.....
Accesories. Alternator water pump....belts
loose covers or bolts...
Harmonics...like a tuning fork, at a certain vibration something starts to make noise and it may not be the engine.
motor mounts...
pipes...
pilot bearing...
something in the clutch assembly
cracked flywheel or flex plate

I too think you are on the right track but make sure it's not...silly stuff.
flex plate flywheel crack....

 

Fuel pumps have a damper spring on the lever. If the spring breaks it will make a noticeable tapping sound. Easy to isolate by stethoscope or a long screwdriver with the handle on your ear and the tip on the top most part of the fuel pump.

 

I’ve never looked at clutch/flywheel yet so maybe somethings weird in there. Shouldn’t be exhaust related. Just put on whole new system with headers and copper header gaskets. I’ve run it without the belt and still made noise. By harmonics, like could the harmonic balancer be bad?

 

It is a newer fuel pump I put on it, but could be a possibility. I’ll check it out also.

 

Those harmonics can really throw you. I thought something broke in my transmission from the grinding noise and vibration. It was a wheel bearing going bad.