I am totally with you. I can't stand it. My dad and brother both do it and not only does it get on my nerves it really grosses me out. I yell at them all the time.
I'd just...say it. Hm. I guess if you were just like "I'm not trying to be mean, but could you chew a little quieter?" haha. They probably don't even realize they do it.
My best friend does it. We've been best friends forever and I've never said anything to her about it. about a year ago, I've started glaring at her everytime she does it, until eventually she was like "WHAT is your problem?" and then I told her. and now whenever we're eating together all I have to do is look at her lol.
I always joke that every first date I ever have will be to a restaraunt, cause if the guy can pass the chewing test then we'll see where it goes.