The scary noises in minecraft are ambient noises that are sometimes triggered by being near spaces that are dark enough for mobs to spawn that are at least 3x3x3. It does not necessarily mean you are close to dungeons/mob spawners or ores. It is usually from a cave underground, but it is possible for your own structures to trigger that noise, if they are too dark.
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They are noises in the code that they make you feel scared as if you were in a dark cave
It means there is a cave nearby or a open terrain...
It depends on what mod you are using, it could use 'scary' noises. Normally the sounds play when you are near large underground caves.
No, but it can be sometimes when you turn around and you come face to face with a creeper. Overall, it's not scary though.
As of the current date of this answer, the next version is Minecraft 1,4, the Pretty Scary Update, which will be released on or around October 31st, which is Halloween.
It doesn't mean anything specific 'in Minecraft', it is short for Major League Gaming.