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1987 Ford F-250 Transmission Rattle

I have a 1987 F-250 6.9 Diesel 4x4. I have a loud rattle at in Park, Neutral, and Drive but not in Reverse. The rattle is never in Reverse. The truck sounds normal only in reverse and not in any other gear even park. The noise also seems to go away at a higher RPM like 1000. The Transmission fluid is full and looks new.

Someone mentioned it could be a cracked flywheel -does this make sense or could there be another reason?