One of my buddies and I decided to visit a Cavern last Saturday. I recently bought a few used reg. which I wanted to try out. My buddy is not a cave fan, but he enjoys caverns, so we decided to get in the cavern section of and stay in the cave entrance. We went thoroughly over bubble check and air sharing drills in case something went wrong. As we reached the cavern entrance my buddy tried to enter the cavern, but could not really get inside (due to high flow). I turned upside down and crawling from the ceiling for about 5 feet, I showed him an easier way to get inside; this was the moment when I first started hearing my second stage sounding funny. I tried to adjust it, while my buddy was crawling at the sealing. He started complaining that water had entered his second stage. Those two facts made me decide that we should stay out of that cavern for the moment being. I asked my buddy to check if he could see any bubbles coming out of my second stage while I was holding my breath. His answer was negative. I took the second stage out of my mouth and hit it with my other hand. The sound stopped, so I gave OK to my buddy and he started leading. As soon as I took my first breath, the noise started again. Now, it clearly sounds as the reg. is breathing, but the point is that I am not breathing. It did not take more that two or three breaths before my primary second stage started free-flowing. I tried to adjust or stop it but no luck. I immediately, switched on my back up which started free-flowing too. WOW, I did not expect that. I always keep my backup on Pre-dive position in order to avoid free-flowing during the dive. At this moment, I am holding both second stages in my hand, and trying to make them stop free-flowing. My buddy, by now had heard all the unusual noises and turned around (later on the surface he told me “I turned around and saw you holding your second stages in your hands, but nothing in your mouth”). He donated his primary and switched to his back up. I turned off my tanks, and we ascended with no other problems. I think I was pretty calm during the whole time, but I do not know how calm I would have been if we were in the cavern zone, or diving in another cave. Moral of the story for me? Make you sure your gear works properly.