when i was a kid my mum's best friend snapped her gum and i thought it was the coolest thing. i asked her to teach me and she refused. she told me she hated that she did it but she sort of couldn't chew gum and not do it, it wasn't a conscious thing she was doing. so the only way to not do it was to not chew gum, which she didn't do while working since she was a bell telephone operator and kinda couldn't do that and work. i think the coworker kinda can't chew gum and not snap it, it is sort of reflexive to her. therefore, the gum needs to stay at home.

the noise would bother me a lot too. then again i'm sitting in my office with my door shut to the rest of the folks 99.9% of the day. and 2 days a week i'm at home working where my cat has yet to develop any annoying habit like gum snapping.

wish i had an answer for you, i'd be just as bothered i've got to say.