I think I know things like this, but I may be crazy

Have you ever listened to Bud Powell or Thelonious Monk and wondered, "what is that weird moaning/groaning sound I hear from time to time?" I have, and I think it's just Bud and Thelonious making involuntary noises while they rip the keys off that piano. Like a hacky sack player who can't control the spastic movements of his or her hands, they just moan and groan while they're playing. Or that's my theory.