WASHINGTON—The classic 1979 sci-fi horror film “Alien” was advertised with the memorable tagline, “In space no can hear you scream.” It did not say anything about humming. Instruments aboard NASA’s Voyager 1 spacecraft, which nine years ago exited our solar system’s outer reaches, have detected a faint monotonous hum caused by the constant vibrations of the small amounts of gas found in the near-emptiness of interstellar space, scientists said. It essentially represents the background noise present in the vast expanse between star systems. These vibrations, called persistent plasma waves, were identified at radio frequencies in a narrow bandwidth during a three-year period as Voyager 1 traverses interstellar space. “The persistent plasma waves that we’ve just discovered are far too weak to actually hear with the human ear. If we could hear it, it would sound like a single steady note, playing constantly but changing very slightly over time,” said Stella …