Is the floating oil pump a noise possibility?
I've never built a stovebolt, but when I looked in Motors just now, it looks a lot like the Model A Ford in there. And when I read "floating oil pump", I remembered when I fixed a noisy Model A engine once like that - the oil system parts were banging around in there. Some jiggling fixed it - no more noise.
Same scenario - this "A" engine had just been rebuilt (not overrevved though). It purred like a kitten but made this clunking noise - it was down there in the oil system.
Just a thought, wish I could offer more. I hate mystery noises!
