Have you ever run your finger along the edge of the bristles of a comb? It emits a very distinct and unique sound that may remind you of dolphin.
Apparently, it sounds like a dolphin call to actual dolphins as well!
In this year old video by Weirdiosity, a man flicks a comb by the underground dolphin tank. Incredibly, at least two dolphins swims up right next to the comb, apparently listening with great intent. One even seems to smile!