In Oaxaca, there are sound-signatures in the air. At first, they add to the mystery of the place and its texture. Later, they come to convey everyday messages, adding a layer to your enjoyment—or irritation.

There’s a certain horn the gas truck uses, which sounds like a cow mooing, followed by a micro-jingle that goes “Gas de Oaxaca.” Very self-explanatory. When the truck is too far away to make out the jingle or the words, you can always make out that electronic bleating—muuuu—and you know the gas truck is near, loaded with rattling tanks and ready to fuel your stove.

There’s an ice cream cart that plays a slow, sentimental piano tune, recorded on a loop, again and again. It caused an old gringo I know, who has lived in Oaxaca for decades, to recoil in horror when it came by, though I have yet to find it super annoying.

There are countless of these sound marks in Oaxaca, and sometimes it’s just as simple as a word delivered in a certain tone. At an apartment I stayed in back in December, every so many days in the early afternoon, the water man would come by and shout “hay agua.” Always in the same tone, always the same words. He’d repeat it once or twice—hay agua—and if no one needed agua, he’d leave.

I have my favorites: the potato steamer’s whistle, and the knife sharpener’s song.

The steamer’s whistle was driving me crazy with curiosity for about three months after arriving in Oaxaca. I heard what sounded exactly like a steam-powered train whistle, just after dark, weird and lonely and somewhere nearby on the streets south and east of the Zocalo, a working class neighborhood filled with screenprinting and letterpress shops.

Then some weeknight after dark, I caught a glimpse of an odd cart: hand drawn, with a tin smokestack and a thin youth bent over the tin belly underneath the chimney, feeding wood into a tiny door. He shut the door and pulled a wire and the machine did its magic. A lid on the top opened, steam belched out and there came the whistle.

I was with Aurora and Mazzy. We stopped at the cart and bought sweet potatoes and banana, steamed and tender and wrapped in foil. And there it was, an end to one sonic enigma, with a reality at the end of it better than anything I could have dreamed up.

Then there’s the knife sharpener’s song. Again, it took months to uncover what this sound was. I’d been hearing, intermittently, a simple tune played on what sounded like a small flute. A short series of notes ascending, then pausing, then descending. Then another pause, and it would repeat.

Aurora’s brother Yukio was visiting, and we were walking down Garcia Vigil near Xochimilco. On turning into one of the beautiful little side streets, cobbled and narrow and sloping uphill, we heard the tune. Yukio started recording, and we fell silent and followed. The sound was coming from a man with a bicycle.

He walked the bicycle, one hand on its handlebars, the other hand holding the whistle. The bicycle was, on closer examination, not just a bicycle. It was a knife sharpener. There was a sharpening stone mounted on it, which the pedals could be made to turn.

Yukio’s recording is posted above. And, if you’re interested in urban sound, you should check out his blog, Berlincast, where for years he’s been making sound maps of the city of Berlin.