As an incessant gum chewer (there are always several packs of Orbit on hand in my desk drawers), I have never understood people who chew gum with their mouths hanging open. It annoys the hell out of me. I don’t want to hear you chomping on your gum! And I don’t need to see it in your mouth!! Some people even go a step further and somehow use their tongue to move the gum from one side of their mouth from the other (of course, with their mouths still hanging open), which is also repulsive. It takes every ounce of self control to squelch my Highly Sensitive tendencies and refrain from whacking these people in the face, if for no other reason than it will make them either spit out or choke on their gum.
In the same vein, gum snapping also drives me to distraction. Especially at work. You are not a twelve year old hanging out at the mall! This is a place of business! You are supposed to be a professional adult! Please act like one. Thank you.