Back in the late 60s Popular Electronics ran an article on two annoying noisemakers. I built a few of "The Drip" -- a device that, as I built it (not original parts values), would emit a faint sound like a kitten's mewing for about a second or two, then go quiet for several minutes. It was a clever little circuit, and the article said it would run for a long time.
It did.
5 to 6 years later, one quiet night, I heard one. Took me two days to find it.