I would say a mantis shrimp, but the fact you have a frog's croak sound baffles me.

I too have the knocks, I have had it since the beginning, so I think I got a mantis from the LR, never seen it, nor have I ever seen any piles of sand. So I am thinking it lives on the backside of my rockwork.

The Frog's Croak really baffles me, I don't shrimp croak!

I get the loud clicks/knocks coming from the rocks in my sump. I've never seen the source. I've always been curious though.

I suppose frog's croak was the wrong way to describe it. I mean that it sounds like more than just two hard surfaces tapping together, there's a resonance to it that gives it an extra "something". It might just be the echo the sound makes bouncing around the inside of the tank.

At first I was terrified that it was something banging on the glass of the aquarium and that it might break, but I couldn't reproduce the sound by knocking against the glass with anything (I probably looked like an idiot trying to do it too).

I suppose I shouldn't worry about it unless things start disappearing.

Do you have Bangii cardinals?. After I sold all the fish from my tank and got a little bored of looking at corals I bought two Bangii and ever since I have heard a similar sounds...Best I can describe is it like hearing from a distance a large marble banging around in a glass jar filled with water.. kind of a muffled sound. I think I have two males and the sound seems be around the time one fish is pestering the other.

I get a double click throughout the day, lights on, lights off doesnt matter and its always Click Click then nothing, sounds like its near where my spray bar is but I have never seen sand piles or anything even after lights out I have checked.

No dead snails or shrimp either so i dont know what it could be

Oddly enough, I see both of mine all the time... if I don't see them when I look in the tank, I start to get a little worried. One is paired with a yellow watchman, the other is a lone soldier. It will definetly get more outgoing though if you throw a shrimp goby in there with it... IF it's a pistol.

any damsles in your tank my 3 stripe croaked / clicked at other fish in my tank all the time

A very small pistol can make a very big noise , I was running a canister filter when I first started my tank & one day when cleaning the filter I came across a pistol shrimp that was about 3/8 -1/2 of an inch . I put him in a 10gal that I have set up & ever since the snapping started in that tank too , & it is loud enough to be heard all through the house at night. Also turbo snails can make a very similar noise by spinning there shells back & forth quickly & hitting the glass . I have witnessed this a number of times.

Mystery solved!

During the day, my turbo snails cluster against the glass beside the heater in the corner where it's shaded. I was finally looking right at one when it puffed it's shell far away from it's body and started spinning it rapidly. Now that I know to watch, I've seen them do it a couple of times. When the shell only knocks against other snails (Is it a territorial thing?), it makes a softer, more muted noise. When it's right against the glass, the knocking sounds like the tank is about to bust open.

It sounds like the sounds is coming from 2 feet from where the snails are making the noise. Acoustics are nuts.