I'm not sure about when a bunny passes gas, if there is actually a sound loud enough to hear. But I think because they can have a hard time passing it, that gas can sometimes cause serious painful problems, which can be heard with extra loud gurgling within their stomach (louder than their normal gurgling). Many times their tummy will feel tight and bloated. And they sit in a position that shows their discomfort.
Hopefully that will never happen to your or anyone's bunny here, but if it does, if not alleviated, a bunny can stop eating or even going into shock due to the stress and pain of severe gas. There are ways to try and alleviate this - here are some tips from the House Rabbit Society.
Administer - 1 - 3 cc of pediatric simethicone drops.
.Begin very gentle abdominal massage. Place the bunny on a secure countertop and start very gradually, rubbing the sides of the belly, and gradually working your way deeper by actually holding the belly in one hand and gently kneading it in both forward and backward directions. If bunny shows any sign of pain, back off and massage more gently. It also helps to periodically raise the bunny up on his hindquarters as you massage, so gas bubbles can move about more freely. You can also do the reverse, carefully lifting his hindquarters into the air while gently massaging, being careful to not let him kick, jump, or hurt himself.
I also offer a little pedialyte to help keep them hydrated which will help pass the gas along.
My understanding is that if the gas or bloat doesn't go away within a few hours, then a vet needs to be called asap.