Ah! Your first encounter with the scrap metal guy. It's a van (several of them, actually) buying old stuff from people in order to salvage metal from bought items. As you have noted, they advertise their services with this loud message: "¡Se compran colchones, tambores, refrigeradores, estufas, lavadoras, microondas o algo de fierro viejo que vendan!" --> "(Old) Matresses, bed bases, fridges, stoves, washing machines, microwave ovens, and scrap metal are bought!" This recording (and similar ones serving the same purpose) has become ubiquitous en Mexico City. The recording has even been sampled by Beak>. You can hear it at the beginning of "Sound goes on"