I share this pet peeve with my dad. When I was a child, I would always get stern looks or an emphatic "Stop smacking!" from him if I slipped up and chewed with my mouth open; even if it was just for one bite. I learned a good behavior: chewing with my mouth closed. And in return, loathe every sound made and the even the very idea of smacking your food.
So here I am, sitting in the hallway of the fourth floor of the JFSB waiting for my class to start, with some guy about twenty feet away (yes it's that loud) smacking away at his pasta or salad or whatever it is, wishing that I had my headphones to drown out the awful noise.