Hospital Haunting

I have had a few "paranormal" experiences in my life, but this one takes the cake as being the scariest experience I have ever had.

My occurrence happened in November 2004 when I was 18. There is an abandoned hospital in my town that closed in March of 2003 from not being up to health standards. My best friend, Brynn and I had been talking for ages about sneaking into the building and seeing if it was haunted as we both had a strong feeling it was. As it turns out we were right.
It was mid November when we sneaked into the building, which had been roped off for safety reasons I suppose, and walked around. It was night time and the only lights we had were our floodlight flashlights. It was freezing so we were bundled in coats and caps and lugging our sleeping bags and backpacks of snacks with us. We had entered through the old Emergency room entrance and after a lot of fumbling around, found the stairs that went through the building (the elevators were useless since there was no electricity). We walked up the stairs, joking around like teenaged kids do.
We decided to make camp on the second floor which was the old maternity ward. We were up there maybe 45 minutes, just long enough to get our bags and stuff laid out before the trouble started.
Brynn and I were stuffing our faces with Ding Dongs when I heard a sound. It was very faint and light, but I could just make it out to be the sound of an infant crying. I asked Brynn if she had heard the crying too. She looked at me like I was crazy and said no. I listened again, it was quiet and the only sound I could hear was the usual car passing ever so often on the street below. I shook off the crying as the product of an overactive imagination and maybe one too many candy bars.
After our snack, we went and walked around, just looking at the left over equipment and goofing around for a while pretending each of us was going into labor. Eventually, Brynn had to use the bathroom so we found one on the other side of the floor which was the only one that wasn't locked. I went in and held my flashlight while she went. We were pretty quiet because we were freaked out to be in a windowless room. Just to mess with Brynn, I turned my light out. She said "Ha Ha, funny Tiff" and giggled. As I started laughing, Brynn let out a huge scream. I turned on my light in a hurry. She came running out the stall pulling her jeans up. She asked me if I had grabbed her leg. Of course I hadn't and said I had been across the other side of the room the whole time. We argued for a few moments, when a loud BANG! made us jump. We didn't say anything, we just high tailed it out of the bathroom, picked up our things and left the hospital.
We haven't been back since and I get chills every time I drive past it.