What makes a Baaah noise in Minecraft?

That noise happens when you're in or near a cave system. If you're mining and you hear that noise you're most likely within 24 blocks of a cave.

Or someone could have spawned a wither if you're playing on a multiplayer server.

It may either be:
1. A zombie lurking in a cave.
Or (most likely):
2. The eery noise/ambient noise that occurs when in deep caves.
You can easily disable it by pressing the escape key or going to options on the
main page, going to Music & Sound Options then changing the Ambient/Environment metre to however much of it you wish for there to be.

If this doesn't work, you must replace the environment sound's file with nothing, or make it empty.
Search a tutorial if you want.