Snapping sounds in my tank

[Toolman]

I just added about 80 lbs of live rock to my tank and now once in a while I hear snapping sounds comming from my tank. They seem to come in fast groups of 3 , I had a mantis shrimp hitch hiker from some other rock but I got rid of it. I have a fidler crab, could he be making the noises. I have lost 3 fish since I put in the new rock. I have a very small crab which I believe is a gorrila crab living in one of the rocks, but it seems to small to be killing my fish. Any body know what makes a loud sharp snapping sound ???