Not sure which one because whenever I go over to inspect it stops
.
Its kind of a hollow grunting noise. Repeated over and over again for a while and then stops....kind of like a deep seal noise
.
Hard to explain. 
They are eating hay, it kind of matches the chews iykwim not sure I do . 
Never heard it before, normal or not?
As I type I am starting to think its Big Pig....