I’ve had some pretty good luck with saying, “Boy, that gum really sounds good!” They seem to catch on right away and apologize, and sometimes even get rid of the gum, but I have to initiate this early, while I can still chuckle a little as I say it. Otherwise, I’d be bonkers and really hostile.
Now there;s a phrase that really irritates me, “Thanks in advance”. I’m not sure why, but it may be a holdover from those tiresome “thanks for not smoking” stickers people liked to put up even in buildings where smoking was prohibited everywhere. Yeah, maybe that’s it. 
Wait, wait, there’s an OP here. Sorry.
Actually, Miss Manners answered a similar question by saying that sometimes you just gotta put up with rudeness. She has a pretty low opinion of “Manners Police”. I agree, mostly.
Chomping noises give you a headache?
Peace,
mangeorge
My girlfriend was waiting to have a procedure for which she had been required to fast for well, much longer than she’s used to not eating. She was very, very hungry. Another woman came into the waiting room and pulled out an entire Mexican dinner - entree, chips, guacamole, salsa - you name it - and proceeded to methodically and unself-consciously crunch, slurp and munch her fragrant way through the entire thing. Besides the receptionist, we were the only three in the room. I got a crick in my neck from keeping my head down to avoid laughing when I glimpsed the look I knew was on my girlfriend’s face. She was radiating hostility and this woman was completely oblivious. :eek:
I kinda wish she had said “You don’t mind if I have some of that do you?” while simultaneously reaching for a chip…
I would have been polite while inwardly hating their lack of politeness.
Being polite sometimes means having to ignore those things that hack you off. We can’t dictate other peoples behaviour (manners wise) no matter how obnoxious it is.
My best friend slurps coffee like she is drinking through a vaccum cleaner…I ignore it because that is the polite thing to do. One day she will realise slurping isn’t polite and we will both be happy.