pistol/mantis shrimp??

This is popping noise is driving me crazy!!! For a while I thought it was just the thermometer bobbing against the glass. When the lights went off tonight there was lots of popping and cracking noises for about 20 minutes. Sometimes it was a small crack like the sound of glass tapping glass, other times it sounded like a big stick being snapped. Also, I would hear a double snap.

I want to put peppermint shrimp in my tank, but I'm afraid that if it's a mantis, my peppies won't last long. How do I catch this thing? I only have a 15g long and there aren't many places to hide. Still, I've watched my tank for hours and have yet to spot anything living besides my corals, pincussion urchin, zebra hermit, and worms.

Well I tapped on the glass a whole bunch and told it to hush so I could go to sleep. It seemed to do the trick cause I haven't heard a peep or a pop for over an hour.