Well, discussing bodily functions I personally have nothing against, so long as it's handled maturely. Bodily-functions-to-be-annoying-for-amusement, on the other hand... no, just... no. It's normal enough for children, but when adults do it (unless it's among a group of close friends who are cool with that sort of thing) are just irritating.
As for the bodily functions themselves, I find a nice belch to be rather pleasing (in time or with curiosity, you may know why, but for now I settle for subtle hints) and farts are not terribly good when intentionally done around other people, though the accidental ones can be rather hilarious, such as when the manager let one fly while ringing up a customer where I used to work, or the discovery I made with my roommates that the baseline of "Speed Garage" music does, indeed, sound like someone farting into a pillow.
Also, where other bodily functions are concerned, I really wish there weren't so many social stigmata against discussing them, as it's rather difficult to understand them well with only one's self as a reference.