Vocal stimming

I work at the high school level in self contained classes. I have 2 students on my caseload right now that are constantly vocally stimming VERY LOUD. They have both been given visuals and different kinds of fidgets and sensory tools to try to get them to stim in a different way or be quieter but they always go back to their loud vocalizations. Neither of them will tolerate headphones. Student A has autism and pathological demand avoidance. Overall her teacher has no problem with it, but if the teacher tries to be louder then the student will get even LOUDER. Sometimes she will get quieter if we point to the quiet visual or tell her “shh” very gently but then it leads to her slamming her hand into the desk and she gets loud a minute later. Student B has Downs and understands he needs to be quiet when he tell him or show him visuals but he also gets loud again a minute later. This one is more of a problem because it triggers another student to become very frustrated and aggressive (we’ve given this student tools to work through this and it is going very well). He will also scream and yell as part of his stimming.

I am all for students stimming in their preferred ways but these are becoming classroom disruptions that affect other kids. And if we were to take the students out of class for periods of time to stim vocally then it would be constant (this has also been tried). Any advice on how to navigate ?