> To listen, go to bostonglobe.com/soundtrack. “The sound,” reader David O’Connor tells me, “will make you shiver.” Not in a Blair Witch kind of way, in a cold kind of way. He’s talking about the metallic ping you hear from those elevated electrical wires in Cambridge when the T’s trackless trolleys glide by. You know — the buses that look like marionettes, tethered to their masters. I knew instantly what O’Connor was referring to, but it’s always fascinating to learn how people associate sounds with different things. For O’Connor, now of Bourne, the noise stirs memories of frigid winter nights decades ago, when he lived in Huron Village. As the No. 72 bus came down a hill on Huron Avenue, he says, the wires sometimes twanged like an out-of-tune guitar; on other occasions, the tone resembled “a lightly struck wine glass.” The colder it was, the sharper the sound. “For me, it was part of the texture of winter,” he says. When I went out to Huron and to Harvard Square on a recent day to hear it, I was reminded, also, of a laser gun in a video game or film. The sound, thin and linear, began one place and ended up another.