Try replacing your mechanical fuel pump.
Had one of those making a knock on a small block. The knock went though the whole engine making it sound like a rod or exhaust manifold leak, but not.
The arm going into the fuel pump was loose where it met the diaphragm plunger. The new pump was not loose there, and the noise went away.
Found it by putting a long 3/8 extension at my ear with the other end on the fuel pump, that was it.
