Clicking sounds in my tank...

[Quinn]

Something like a machine gun in the distance, if you've ever heard one, short burst, maybe 3-4 clicks, max. It's been going on for about a week now, but generally at night. I have a feeling it's a pistol shrimp of course but I would have expected to have seen him already. Could it be anything else? I had two mantis shrimps when I got my live rock, but I don't feel that this is a third because of the noises, apparently mantis shrimps have more clicks in chunks, a few at a time, rather than one-offs. Any thoughts? I'd just like to see the fellow, know exactly what it is.