As soon the train doors opened I heard the noise. That had to be it. Just as I stepped out of the train it stopped. I wandered around the inside of the station for a few minutes, but it had gone quiet. Then as I went through the turnstiles it sounded again. I still couldn't lcoate the source though. I left the station and walked around the Balboa Park area a bit in search of a conevience store. Man, there really isn't much there. After a few minutes of pointless wandering I headed back into the station, defeated. Then as I was riding the escalator down to the platform I heard it. It was close this time. There was a small green speaker emitting the sound, it wasn't a creature at all. It was some sort of ghost in the machine.