I still hear the clicking quite frequently. It is very loud, and if my hand is in the tank when it happens, I can feel it through the water. I haven't been able to localize it yet, but haven't seen a mantis for over a month in the main display. I did add a small pistol shrimp that came as a hitchhiker on some new rock. I found it in the bottom of the bucket after I added the rock. It was about 1/4 inch long, and had one huge claw and one normal sized one. This little 1/4 fellow could sure make a racket though. In the bucket, he sounded just like the much larger mantis shrimp. Since adding him to the main display, I haven't seen him since, but I still hear the tapping noise. It could very well be, that I have a few of these small pistol shrimp that don't come out too often, but are making the racket. Otherwise, I could't imagine the small blue-legged hermits I have making the loud popping noise that I frequently hear.
Andy