Ambience

Ambience (sometimes called cave noises) is a set of sounds that occasionally plays when the player is in close proximity to an unlit cave. Despite their name, they are not sounds that one would expect to hear inside of a cave, such as falling pebbles, but rather supernatural noises frequently likened to ghostly train whistles, motorcycle engines, demon roars, and screeching violins.

Ambient sounds require that a space at least 3×3×3, entirely less than light level eight (with lower light levels being more likely to play the sound), and with no lighting contribution from the sky, be positioned ten to twenty blocks away from the player in any direction. The space does not need to be sealed off - a sound can even play from a space that the player can actually see - nor does the player need to be underground to hear it. The sounds can also come from player-created spaces, even if that space is a box above the clouds.

Trivia

 * Contrary to popular insistence, ambience is not an indicator of nearby ore, dungeons, or mobs. Following them can still lead the player to such things indirectly, since they emanate from unexplored caves.
 * Ambient sounds were created by C418, the same person who composed Minecraft's music.