OMG - I can't believe you started this thread! It's crazy because that is seriously my
*BIGGEST* pet peeve EVER. I had such a difficult time in school because when we would all sit down at the lunch tables to eat, half the kids would chew like this and it absolutely *GROSSED* me out!!!!

Sometimes I couldn't even force myself to eat lunch

And another thing I can't stand is when people chew their food and talk at the same time and you can see in their mouths -
OMG!!!!!! 
Another thing is food being on peoples faces when they eat. I will seriously get nauseous. I know that probably sounds so rude and snobby and I don't mean it to, I swear...but these are things that have always just REALLY bothered me.