Chirping Sounds

Hey guys,
every now and then I hear chirping from my tank. I once heard it before when 2 of them where fighting over a shell, but I got a lot more and all of them seem happy since then. They are currently sleeping/ hanging in there prefered corners and not directly next to each other.

Humidity and temperature is around 80 and the tank and food are to the standards described in the documents here in the subreddit.

I have no idea why they or at least on of them chirp.. Has anyone had similar expieriences?