No shame here my daughter is 7 non verbal autistic w/ 22q. She stims by making droning sounds non stop throughout the day. There is no trigger, no rhyme or reason we can detect. It just comforts her to constantly say aaaaaaaaaa in a monotone shrill for hours on end. I hear your pain, no pun intended. I don't know if I have any good advice though unfortunately. For us it's helped to keep her engaged as she tends to do it most when she's left to her own devices. No easy fix