H.C.
I used to try to distract my son and get him to do something else with his mouth. For example, I would open my mouth wide and say lalalala and try to get him to copy me. I would also have lots of different teethers or things for him to chew on around so he could find something that felt good to chew on instead of grinding.
In the end I do think this is a normal part of them learning how their bodies work and experimenting with new feelings and sounds but it is very hard to watch and listen too. Good luck.