All 4 of ours. It doesn't stop unless they're:
#1: Eating.
#2: Scouting the "new" terrain after their tractor has been moved.
#3: Sitting on the nest box.
#4: Fighting over food.
This is what we get:
In the morning when I go out and feed the dog:
BAAAAAAAAAAAWWWWWKKK!!! If all four start, it is truly deafening.
When I'm observing them: Urk urk urk, bokokawk, *weird gurgling noise,* etc.
After finishing a treat, usually bread: Baaaawk, bawk bawk BAAAAAWWWWWK!!!
All other times: Odd mixture of sounds, including weird chirping noises on occasion. I assume it's whining, as they only shut up if I give them something. They get noisy if I try to hold them.