Just a thought, did ya look for an exhaust manifold leak? Couldn't change the noise killing cylinders via spark plug wires, and seems repetitive. Not to mention, comes and goes?..
Glenn, I've had exhaust leaks that made a slight serious sound before and agree they can be a cause for concern. However when you listen with something touching the block to ear, it's definitely a low frequency metallic sound from deep in the engine.
The above said, if I discover your suggestion is spot on, I'll buy the boiler makers as we toast to your psychic abilities!
RonR
