Clicking Sound

Well whatever it is, it's attacking something. I'd look for broken snail and hermit shells and do a headcount on your fish.
 
could be hitting the LR, or the glass.
 
I'd keep an eye out. Sometimes shrimps can be amazing at hiding. You'll see something eventually.
 
Definitely keep an eye on your inhabitants and your live rock. It is probably knocking out holes in your LR to make its cave. A mantis will eventually reveal himself, a pistol will very rarely if ever come out. But Grins is right, if it is a pistol your cleaner shrimp is in trouble.

I have a pistol now, at first I thought it was a mantis, but then I found him and fed him, could tell right away it wasn't a mantis. I also found the graveyard! Right behind his burrow there is a massive pile of hermit shells, that is how he was feeding, he was eating all my hermits!

In my case I didn't know that I had him and I could never figure out why my hermits would disappear so fast. Anyways, now I feed him frozen silver sides 3 times a week and my hermits can live longer lives.

Good luck.
 
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