One thing you might try before striping the engine (if you have not already) is with the engine running
Is to disconnect the leads one at a time, this is to stop the combustion cycle in whichever lead you have
Disconnected, the engine will run like crap on just 3 cylinders but if the noise stops at least you would
Know it was connected with that cylinder, Big end, Little end, Piston , etc.
The best way to do this is to stop the engine disconnect one lead and restart the engine!…….
This way you minimise the risk of getting a shock !…….
Let me know how you get on.

Steve.