[Mr 42]

I would remove the manifold, and check for valve trouble.
If its not i guess you have to remove the head.

With this said my son had a SBC that started to knock (ping) So we retarded the ignition.
And i went away, but come back he drove it 500 miles with ping(knock) retarding the ignition a couple of times. It was no 1 rod bearing that had given up.
So weird things happen.

But start with the basic checks first.
Remove ignition wire to a cylinder at time to see if the sound changes.