Need help with mystery noise

[asylumdown]

About 5 months ago, I started noticing a strange noise coming from my reef. At first it only happened after the lights went out, but now it sometimes happens during the day as well.

It sounds like a series of sharp, fast knocks, with a resonating quality that makes them sound like a knock mixed with a super sharp frog's croak. Usually 4-6 in a row. Sometimes it's really loud, as though a small hammer is banging on the glass, other times it's quite muffled. You can hear it 20 feet from the aquarium.

I've not added any new live rock in a year, so whatever is making the noise must have pretty much been a larvae when it arrived.

It happens at least once a day, but sometimes 2 or 3 times a day. A couple days ago I was scraping the sides of the glass when it started - it sounded like it was coming from 2 inches in front of my face, but I couldn't see anything. In fact I've never seen anything, it always sounds like it's coming from behind the rockwork (the side I can't see).

I've narrowed it down to either a pistol shrimp or a mantis shrimp. I have lots of hermit crabs, and at least 3 other species of crab that hitch-hiked in, as well as several cleaner shrimp. None of them are disappearing so it makes me think it's not a mantis shrimp? I've never heard a pistol shrimp or a mantis shrimp before though. Any advice?