While mining i saw "eerie noise" appear in the subtitles (new feature of 1.9 showing game sounds; i keep it on as i like my volume low). I am unable to recreate it, although not much time was put into it, and so i assume it is rare and possible for aesthetics.
What causes it? And does it signal anything?
21 Answer
There are a few sounds that could be listed as "eerie", and (from the explanation in the comments) generally appear near undiscovered caves.
From the wiki:[1]
Ambient sounds can play in a space of any size, but they seem to play much more likely in larger spaces positioned 10 - 20 blocks away from the player in any direction, and completely filled with a light level less than 8 without any lighting from the sky (the ambience will more likely play from the direction of lower levels because of a lower light level).
So, while these sounds can occur at any level, in any structure, just so long as the light level is below 8, these "Ambient" sounds can appear.
[1]: This link also has a list of all the "Ambient" sound tracks for comparison.
