The planetary gears can make noise, sometimes quite a bit of noise. You can replace those whilst you rebuild the transmission. I used to have a quiet C-4 till I had a "trans Guy" rebuild mine (circa 1980, also "just for leaks"). My guess is he switched out parts or re-arranged items in the planetary gears differently than before taking it apart. Essentially, the noise never went away from 1980-on. I recently purchased another transmission to use for parts when I rebuild the trans again for my restoration in hopes that the whine will be gone. Besides, I wanted an original date-correct converter rebuilt too (I had exchanged mine back then).