K.N.
My 14 month old does this and it drives me nuts. She has 13 of her teeth now and she can really get them grinding!! The sound is worse then nails on a chalk board.
Just keep giving him something else to munch on when he does this, I think the vibrations it causes feels good when they are teething. They make a vibrating star teether toy that my daughter loves and it usually works as a good distractor. I also noticed when I react to it she does it more!! So try not to make a big deal out of it otherwise it could just become a game. Hopefully your doctor has other ideas for you.