It could be a mantis. It should sound like two marbles being struck together. If you do have one be careful. Like Memnoch said, they can shatter the bones in your finger (with the force of a .22 bullet). Keep an eye out for it. If you see it and notice which rock it has made its home in, take the rock out (very carefully). I forgot how to get it out of the rock, spraying water or hyposality bath or something; do a search on mantis shrimp on this site, there are a lot of posts about them. Then you can either flush it, sell it to the lfs (doubtful that they'll take it), give it away here, or keep it (this is what I did with mine). They are very interesting creatures and easy to keep.
It may turn out that you don't even have a mantis. The crab could be the culprit of the chewed up jaw. I don't know why an attempt at jumping out of the tank would cause its jaw to appear chewed up.
