It happens to me all the time. Depends on the position but if I get tight and J is moving all the way out, which he likes a lot, then air will get trapped.
Sometimes, after a particularly energetic session I'll have quite a lot of air that can come out. J calls me his squeezy toy as he can hug my tummy and make me fanny fart more :P hehe, we're not exactly squeemish!
I've always made noises so got used to it a long time ago - never had a partner yet thats had a problem with it :)