For me, it used to be Disc 11, but now it's that Elder Guardian jumpscare ghost animation sound thing. One day, i was walking around in a server with buggy generation. Little did I know, there was an Ocean Monument hidden under some sand. I was just walking, and then I saw the ghost, with the loud sound and all. When you're expecting it, it's not that scary, but it almost gives me a heart attack when I have no idea it's coming. Also, when a player ender-pearls to me on a server when my back is turned.