Sounds like a worn or warped input shaft.
Considering you unbolted the convertor and the noise stopped.
Other than the sprag or convertor itself (which you said you changed to a new one), the input shaft is the only thing that would spin while in PRN or D.
Check the shaft or the bearings it rides on.
Sometimes they wear prematurely and dont lubricate the shafts the way they are intended to.
Other than that, I'm stumped.
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