So M. was kept up much of Christmas Eve by an irregular sharp tapping noise that she thought was an animal in the ceiling of the next room. The noise happened only when all the lights were out and stopped if she turned on a light.
Last night she went to bed while I began watching a movie when the noise started again. I poked my head through an access hole into the rafters of the ceiling feeling very much like the Captain in Alien (1979). Nothing.
I sat in the dark waiting. A series of sharp almost metallic or rock-like clicks, inconsistent in number and duration with minutes between sets, happened. I quietly moved and waited again to triangulate the noise.
The rapping happened again and I spiked the location with a tight-beam spotlight - to find that the lone fish in the aquarium had discovered, after a couple of years of residence, that it could headbutt the glass thermometer (suspended inside the tank by a string) against the inside of the aquarium's glass wall. Moved the thermometer and the noise didn't recur for the rest of the night.