I've done quite a bit of testing for noise.
I found any shape that has a trailing edge starts out the loudest.
The quietest have rounded backs ( para, moon, t-hawk)
Also the shorter in height the better.
Also the more twist in the feather the louder.
Once I found the best combo of looks and quietness in a feather for my taste, while still having a good amount of feather back there to do the job ( ended up a slightly modified 4x 5" T-hawk chop).
They were silent in flight and more than enough feather to forgive my mistakes a little.
it took me quite a while to come up with this "perfect" combo.
After shooting a while the feathers got a bit worn and made noise anyway...lol
moral...shoot what you like...lol