I am not vouching for this info, just passing it along. check out
"A lot of people seem to be confused about this. Pistol shrimps, which are "generally" viewed as non-harmful to most inhabitants of the tank, can and do make these clicking noises using one of their enlarged, specialized claws. Mantis shrimps cannot by themselves produce such noises. However, they can and do produce clicking noises whenever they use their raptorial appendages to hammer something hard, whether it be the armored shell of some luckless crab or mollusk, or the walls of their cavities. I have read somewheres that one way to tell from the clicking noises is that pistol shrimps tend to snap in ones and twos, whereas mantis shrimps make rapid multiple hits.
The point here is that clicking noises don't prove anything. However, in conjunction with the continued loss of inhabitants, and the lack of any other viable culprit, you can probably assume that you do have a mantis problem. "