I have heard my hermies chirp on a few occasions. One was in the car on the way home from being rescued, a few times during "fights", and a few times randomly at night while newbies were getting settled into iso. A few weeks ago something happened that I thought was intriguing. i have 2 tanks, one has a built in pool and waterfall. The other is more arboreal and larger. I was doing deep cleans and rearranging. I moved all my crabs into the larger tank once I had finished it. I had to wait to do the smaller tank because I knew there was a molter in there. A few nights later the molter dug up. I decided to leave him there alone for a while to make sure he was hardened and up to full strength before going back with the colony. Every night he started croaking like crazy. Night after night! Finally, I went and picked a gentle crab that was smaller than him and added her to his tank. That solved it; he was looking for his friends! I don't recall having read that was a reason they chirped, but I think he must have been calling for the other crabs. He's never lived alone before.