A pitcher makes a very loud noise, that she tries to make come off as a grunt. She makes this noise well after the pitch is delivered, when the batter is starting their swing to be exact. Is this legal?
I have always found that nonsense annoying, but quite frankly, have never been able to get that to stop. The umpire will view the grunting or slapping of the glove to the leg to make a loud popping sound as a timing mechanism used by the pitcher and a normal part of the wind up. But note that it must be part of the pitcher's natural motion.
If, on the other hand, it is perceived as an act only to distract the batter since it is long after the release, than you may be able to get the umpire to get the pitcher to stop. Not likely, however.
Your best course of action, however, is teaching your players to learn to ignore all the extra curricular activities that a pitcher events and instead focus on the pitcher's hand and release of the ball. This form of mental toughness and single mindedness will always beat the glove slapping and/or grunting done to disrupt.
If any other coaches has a view on this, I welcome your input.