It's usually just a slightly embarassing chuckle so don't worry too much. It might have as much to do with your position as anything. Doggie style seems to produce more air than missionary. And the level of energetic air being forced in via piston is a huge factor. (as clean as I could make it)
I wouldn't imagine you being kicked out of bed for it so smile demurely and excuse yourself. Really most girls get at least 'some' if their partner is energetic enough. Try a slow sweet session and see if that produces different results.
i think it is caused by being nervous about it happening and the position which you're in during sex. I also think it is something that some women just do and I agree you might want to google it. it also is due to air being ushed out of the vagina during sex I think? I'm no expert I just heard of a similar story on loveline once.
Google it for answers, you'll probably have better luck, or just go to the doctors. As for the turnoff, I can't imagine I'd want that to happen while I had sex. At the very least the thought of it seems pretty gross, though I've never heard it so I can't speak for how it is during sex.