Not a power line, but, a cable hung over the highway for hanging the traffic standards.
Just as the Stearmans nose crosses into the intersection, there is a sharp “click” in the audio, and something dark and rectangular can be clearly seen falling from beneath the aircraft. Viewing frame-to-frame, the object visually become smaller on its trajectory away from the aircraft. The object is tiny by the time it is last viewable near the left corner of the second from the left hanging traffic light above the police officer. Whatever that object is, I’d speculate that this object was a projectile moving horizontally away from the port side of the aircraft.