I think it is some kind of machine - be it space alien or construction of the 'other' people on the island.
I think it was the first episode, when they heard the 'thing' out in the jungle, the black lady (I can't remember her name) said that it sounded like something she knew but couldn't pinpoint. Someone asked her where she was from and she said New York City.
I didn't hear that the first time. I heard it in the recent rerun.