Ok so growing up I was always told stuff like "Don't put your elbows on the table" or "Don't play with your food". Some of the rules were kind of weird looking back, but I grew up being taught that that stuff was was considered rude.
There are some things that actually make sense, like swallowing your food before talking, or just chewing with your mouth closed. Nothing bothers me more than when someone chews like a fucking cow at the table, or to a much lesser extent, over the mic in voice chat. (Misophonia's a bitch...)
Rant below