personally i have always heard multiples.
i'm thinkin it maybe something else.
????metal shifting in the lighting, the bulb cooling down, syphon bubble from a vortex of water (sounds like a hand slap onto water), snail, sometimes my tangs will wack the tank with there tail while posturing, crab snapping it's pinsers as it picks the rock, i've even had my lawnmower blenny snapping the water as he eats the alga off the eurobraceing. i wouldn't panic just yet, keep investigating.
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