When can you start Hearing peeps

[Suess hens]

I have heard you mention being able to hear them inside the shells. I was wondering when you might be able to hear this.
Our cat was flipping out a few days ago and staring at the incubator. Just curious if she was hearing anything or just being a cat...she can't see what is in there she hasn't been on top and has no way to the eggs inside from anywhere around.

 

[chanda]

I've only noticed it the day of hatching, as they prepare to pip and afterwards.