Originally posted by TangoBlue:

- In the swamp, don't trust what you hear. It might be water lapping against the shore, it might be a blob bouncing near you.

This is true of almost all biomes. They have an "Atmosphere sound track" that may sound like something happening nearby but not actually having a mob near. i.e. the sound of a neck near the coast in the meadows, rattling bones in a burial crypt, lapping water sounds in the swamp. You can't always trust your ears because of it.