From
now until Metatron and Lucifer's official return to their respective headquarters in a few days, characters will be able to hear mysterious, occasional shrieks that echo hauntingly throughout the realm.
The real reason behind it, which no character will know (though those who can sense things and get visions, like Blythe, may get occasional images of a creature being formed, struggling into existence, but not from a womb... ) is that Sphinx, a character for the trial of the mind, is currently being created by Metatron and Lucifer.
These sounds may sound like a variety of things to people, but the closest way to describe the noises are like haunting bird shrieks that make you want to glance over your shoulder just to make sure there aren't any monsters behind you. Characters can liken the sounds to other things, I just thought it might help if I tried to give some kind of vague description.
Have fun with it. =3
Balam has started to notice them, and L as well,
here, for those who might be curious.