Well, it would stand to reason that if you got the Pistol shrimp 2 days ago, and for the last 2 days you've been hearing this click, it would be him, right?
As for why would he click? That's why he is named a "pistol" shrimp. They make a "shot" sound when they move their pinchers. I was at a LFS
store once when we heard a loud CRACK, and the LFS
guy laughed and said he just got a new pistol shrimp, and it had been doing that all day.
Here's what LiveAquaria has to say about it:
"The sound it makes comes from an appendage on the pincher which moves when the pincher is opened or closed and water is ejected. The Pistol Shrimp can be easily confused with Mantis Shrimp if judged by sound only. Unlike the Mantis Shrimp, it is not harmful and will not pose any threat in the tank, with the possible exception of smaller shrimp."
Hope that helps!
38 Gallon (Freshwater): 1 Green Swordtail, 3 Yo-Yo Loaches, 1 Clown Pleco, 1 Blue Gourami, 4 Otocinclus, 7 Cherry Barbs.
46 Gallon (Saltwater): 60 pounds Lalo Live Rock, 60 pounds aragonite sand, 1 Coral Beauty Angelfish, 1 Peppermint Shrimp, 1 Mushroom Coral, 1 Emerald Crab, various snails and hermit crabs.
10 Gallon (Saltwater quarantine):