Janae Bixby first heard the sound near Pentagon City, where I-395 and Glebe Road intersect, as she picked up her kid from daycare on Monday evening around 5.

She described it as “some sort of clock or doorbell chime that you would hear — very digital.” She assumed the noise was coming from the building and started heading home.

But then, in her car ride home to the southern edge of the Del Ray neighborhood in Alexandria, she kept hearing it, again and again. At first, she figured maybe it was music she had unwittingly put on in her car, but opening her windows made it apparent that the noise was coming from outside.

When she got back home, turns out her husband had been hearing it, too.

Over in Arlington Ridge, right outside of Crystal City, John Bergin was in his backyard when he heard a sound “not loud and oppressive, but very clearly and distinctly and repeatedly, and always the exact same set of tones, like a bad techno doorbell.” His wife heard it from inside too, he says, describing it as “a church that went to digital bells but bought the really cheap recording.”

Across Alexandria and Arlington, residents hopped on social media to see if they were alone in hearing these repeated tones. On NextDoor, some suggested it was a recording of the soundtrack from Close Encounters of the Third Kind, or a military alert system, or a particularly loud ice cream truck, among other ideas.

Never ones content to sit back while a mystery is afoot, DCist clearly had to investigate (though, when it comes to mystifying sounds, we’re not exactly batting a thousand — shout out to the loud boom in Northwest D.C. in April 2020).

One suggestion repeated on NextDoor was that the source of the noise was the Netherlands Carillon, a huge bell tower given to the U.S. by the Netherlands as gesture of friendship after World War II. It’s nestled between Arlington National Cemetery and the Marine Corps War Memorial. With its 50-odd bells, the carillon seemed to have the means to be the repeating chime sound, but did it have the motive?

“No, not the Carillon,” National Parks Service spokesperson Aaron LaRocca answered over email to the question of whether the bells sounded off on Monday evening. “There are bells at Arlington National Cemetery, but those would be hard to hear from Alexandria (as would the Carillon).”

As with so many enigmas in the Greater Washington region, particularly those of an auditory variety, we couldn’t ignore the possibility that this had to do with some kind of alert system being tested. We also received a hot tip that at least one person at Joint Base Anacostia-Bolling heard the chimes coming through a loudspeaker. So, of course, we reached out to the Pentagon.

“We’ve checked, and this does not appear to involve the military,” Pentagon spokesperson Sue Gough said over email. “Recommend you check with local authorities in Virginia and around Bolling.”

And it was at Joint Base Anacostia-Bolling that we finally cracked the case.

“We were actually testing our Giant Voice System on Monday evening,” says Anastazia Clouting, a spokesperson for Anacostia-Bolling.

Turns out, the military installation issued a warning on its Facebook page: “Please be advised, there will be maintenance on the Giant Voice System this afternoon which may cause it to go off. At present, there are no real-world emergencies.” (Presumably, if folks could hear the Giant Voice System from across the river, people closer by were also treated to the sound.)

Clouting confirms that the Giant Voice System, which is what Department of Defense facilities use to send out safety messages, did indeed go off on Monday. “I remember being relieved — at least it’s bells and not, you know, nuclear sirens,” she says. “That can be kind of unnerving when you’re trying to get your work done.”