From http://dev.bukkit.org/bukkit-plugins/fun-hard-mode/tickets/102-suggestion-creepy-noises/
Reported by bigscary:

Suggesting scheduling a task so that players randomly hear the sounds of nearby monsters when they're underground (even when there are none), preferably only once every few minutes to keep them on edge. "I swear I heard a zombie! Where the F is he?!"

Happy Halloween! =D

Eh. Probably better suited for a "prank" plugin