Loud Clicking noise coming out of the tank

[Midway]

Hi there,

I've been hearing this from days coming out of one of my tanks now. its a loud clicking noise, the first time I heard it, I panicked because I thought my tank was cracking or something but it was not. its a loud clicking sound, that's the only word I can think of to describe this loud noise coming from the tank. it happens more often when the lights are out at night though.

Does anybody know what could that be?