I great interview by Nerdist on Star Wars: The Force Awakens' sound design:
http://nerdist.com/kylo-rens-signature-force-sound-was-a-purring-kitty-say-sound-designers/
"Kylo Ren Force rumble—the really chunky and animalistic rumble—is my cat’s purring. It’s heavily pitched and slowed version of my cat’s purr that becomes Kylo Force rumble."