440 head noise. Perpetual problems. Problem solved

RustyRatRod said

That even sounds like a cracked flex plate. Goes away in drive or reverse. Have you tried taking the belts off to rule out something in the belt drive? If the flex plate is broken around the crankshaft bolts, you will not be able to see it......until it gets really bad.

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Usually the first thing people actually see is a rusty line somewhere on the flex plate.:D
1. He got on it hard and all the sudden it's making the noise.
2. It's not there on startup, but gets louder as it warms up.
3. Goes away when put in gear.
4. Sounds like rocks rattling around.

Sure is adding up to a flex plate, and may very well be around the center between bolt holes and can't be seen.
If this were me in this situation I would pull all the converter bolts and push it back then take a good look with a good light.