So I was playing my singleplayer hardcore world when I was running to the water. Suddenly I heard zombies, a lot of them. I quickly thought to myself with my extensive play and knowledge, that I could have stumbled across a mob spawner or a cave. I dug down, but before reaching the cave I heard a very, very creepy laugh. It sounded like a bunch of demonic little girls laughing.

I know about ambient noises, but I have never EVER heard this. I checked all of the sounds, but to no prevail. I found nothing.

Please shed some light on this dark noise,
Thanks,
Splash