440 head noise. Perpetual problems. Problem solved

MAN that sounds like a heat riser.

Oh and people with a lot of experience get tricked by flex plates.

You know, if you have to (and I have) you can run that thing with the V cover removed and feel everything while it's running.
Sometimes it gives you a clue when one rocker rattles more than others and things like that.
I push down on rockers on the valve end, the pushrod end, or even feel pushrods while it running to track things down.

I like to use a section of cardboard about 6 inches tall and as long as the head between the valve springs and the valve cover rail to keep oil from going everywhere.