People eating crunchy foods.
I once suggested that crunchy loud treats should be banned from business meetings. That includes ice for drinks. I can’t tell you how many times I’ve been completely distracted from the business at hand by the maddening sound of some douchebag mindlessly (and open-mouthed) gnawing on ice or chips or something.
My office mate rarely takes a lunch. When she brings in something crunchy to eat at her desk, I have to get up and go away to lunch myself or I will have to commit violence.
IF people close their mouths when they chew, this is not nearly as annoying.