I’m starting to gather examples of curious acoustic effects – and the sound of the El Castillo pyramid in Mexico is a really good example. An article  on this has just appeared in an academic journal I help edit.
Apparently, if you sit at the bottom of the steps you think you hear rain dropping. The effect is caused by sound skimming the surface of the staircase. The sound reflects off the regular pattern of the stairs, creating a very distinctive effect. The sound of one person climbing the steps above you is probably rather like a chirp, but add together many walkers and lots of feet stomping on the stairs, and it sounds like raindrops falling into a bucket.
Was this intentional or accidental? It’s very hard to tell. In the past, human society was less vision-centric. But proving intentional acoustic design in ancient monuments is very difficult. A few academics have been trying to show that our ancestors did deliberately build sound effects into burial mounds and stone circles. But some of the claims are controversial and questioned by others.
 The Acoustic Raindrop Effect at Mexican Pyramids: The Architects’ Homage to the Rain God Chac?, Calleja, JAC; Declercq, NF, ACTA ACUSTICA UNITED WITH ACUSTICA, 95, 5, 849-856, (2009)