TH350 noises, please advise

[MountainGoat1966]

83 C20 with a non lockup TH350.
Transmission was rebuilt for the previous owner of the truck and now I am dealing with the aftermath.
It shifts fine, doesn't slip, and the fluid is not burnt.
However, recently it started making whirring and whining sounds upon the engine starting and the tranny innards begin to spin.
I suspect the rebuilder did a slipshod job and did not replace the bushings and sprags.
Possibly a bad sun gear ?
I need to rebuild it myself, as a shop is totally unaffordable, even if there was one I could trust.
Any tips and advice on doing a rebuild would be greatly appreciated.