I just treated myself to dinner on the way home from work (I go through a big train station on the way home and there are loads of places to eat).
A mother and daughter sat down opposite me and both ordered noodle soup and other bits and pieces. As I was alone I had my headphones on, audiobook on the go full volume.
I honestly could hear the slurping/chewing/mushy noises as they both ate their food while carrying on their conversation through my headphones.
It kind of went:
...chatchatchat...MLAMQUOISHMASHMASHSCHLAP...chatchatchat...SCHLUUUUURPQUISHQUISHMLAM...chatchatchat...
I actually lost my appetite on the spot and normally nothing on earth puts me off my food (I once supportively ate my flat mates cheesy chips for her while she was voming).
There's just something about that "smacking" noise that makes me get furious.
Bizarrely, when an admittedly quite noisy family with several DC came and sat behind them honking with laughter about something, they gave scandalous eye-rolly looks. 
Blergh!!! AIBU? How do people not realise they're doing it??