Yakiimo---

I was walking home from the station and I heard this long, drawn-out cry, 'yakiimo-o-o-o! yakiimo-o-o-o!' with drums beating. It sounded very solemn and very Japanese.

My initial, ridiculous thought was 'is there some kind of religious procession happening? Is this like a call to prayer?'

My second thought was 'no, it's a "song" being broadcast through the neighbourhood, therefore, most likely it's some kind of truck going around collecting garbage. (There is an unnecessarily large number of unexciting trucks which broadcast music to announce their presence.)

Then I realised what they were singing; it was a sweet potato truck, selling hot sweet potatoes baked right there in the truck. Apparently the Japanese equivalent of an ice cream van.
If I hadn't just had dinner five minutes before, I might've gone and tried one.

You can hear the song yourself in this YouTube clip.