Mantis repeatedly hit something, either they're mining the rockwork in which they're going to be mining for awhile, or they're agitated and trying to destroy something (like my powerhead). When they're killing something usually you can associate the sound with something also disappearing.
Pistol shrimp is usually only 1 instance of the noise, and I rarely heard mine ever and the most I heard was 3 in a row shooting my plate coral for some unknown reason. Usually it'll just randomly be a loud click I might notice if talking in the room. Sound pretty much identical to this but rarely as successive.
Worth mentioning mantis hits might sound slightly different eat time just because the angle+distance+surface they attack can add variance. Pistols are pretty much identical each time because they're not contacting anything and its the bubble mostly creating that noise.
And can confirm she does this
all the time
Actually one of my biggest regrets was building this tank in my room because she can pull an all-nighter on the rocks when I have work in 6 hours. Its worse cause I keep getting up to check whats agitating her and its usually the powerhead or she's expanding a narrow spot in the rock she just wants to fit through lol.
Agree with ditching the goby idea, hitchhiking pistols don't typically bond with gobies. There's a very select few alpheus sp. that actually pair, any hitchhiking ones are pretty solitary or one of the colonial sponge groups.
Mantis are easy to find cause you'll see them actually come out, and usually always have their eyes peeping out. It'd likely even come out for mysis floating by.