It's the annoying habit of Chewing with your mouth open..
When I am sitting across or next to someone who is chewing with their mouth open all I can think of is shouting at them 'Close your god damn mouth'. It bubbles up inside me and try to contain my annoyance, but sometimes you just can't. When it's family you need to tell them, as otherwise you will go crazy at every meal time. Situations where it is not appropriate is out in public or a restaurant, or in front of people who you don't know well enough. Remember your surroundings and pick the best point possible to let them know that your driving them crazy.
Chew closed, and chew well.