It’s known as the Charlestown Bridge, also the North Washington Street Bridge. For our purposes, a renaming is in order. On a friend’s suggestion, we’re going to call it the Humming Bridge. With origins in the late 19th century, the span connects downtown Boston to Charlestown over the Charles River. Back in the day, it carried elevated trains, swung open to let boats in and out of the river, and played host to fights between the toughs of Charlestown and the North End. Big sections of the bridge surface are just steel grates, composed of thousands of small squares, the water shimmering beneath them. Which brings us to the hum. If you’re walking on the bridge — this is how the Freedom Trail takes people to Bunker Hill — it sounds like you’re at the edge of the Indianapolis 500 track. As cars zip over the grates in both directions, their tires interact with the steel, producing a loud hum or whine. The faster the car, the higher the pitch. Shut your eyes and you can imagine Indy cars racing by at insane speeds.