ed, i have to question your logic that making comments is heckling but making sounds is fair game. honestly i think making fart sounds is more obnoxious than comments because at least it takes a little wit to come up with a good heckle line and not end up embarrassed.
case in point, i've seen ron white a few times in big-ish venues. he was doing his routine about how he stopped smoking, and then pulled out a cigar. after struggling to bite the end off for a few seconds some wise-ass in the front row yelled out "need a clip, buddy?" ron stops and goes, "yeah, you got one?" a pause, the guy says he doesn't. ron laughs and goes "well that was pretty fuckin pointless then, wasn't it?" big laugh and the guy was quiet the rest of the performance.
also a good example of how a classier comedian deals with a heckler.