Is it a bird? Is it a plane? What is it that's causing a strange humming sound around Strangford Lough?
Baffled residents have been asking that question after a strange noise, lasting more than three hours, was reported in the area after midnight on Friday.
One person took to Twitter to try to figure it out, posting: "Anyone else hear this weird #hum over Strangford Lough area last night? What is it??? It went on for hours."
Ian McConnell (aka @TallowKetch) captured a video of the peculiar humming sound to share on the social media platform and the noise - which sounds like a loud car horn going off - can be heard clearly.
In another twist, the 52-year-old writer, who lives in Kilclief, which is three miles from Strangford and four miles from Ardglass, revealed that he could only hear the hum from outside the house.
"I was letting my cat out in the early hours of Saturday morning when I heard it," he said.
"I didn't hear anything inside but as soon as I opened the door I could hear this noise in the air. It was like a droning or a hum. I don't even know how I would describe it.
"I was immediately struck by it. I've lived here for two years and I'm often in and out at night because of the cat and I've never heard anything like it before, so I was very struck. It was very loud.
"I would have got into the car and driven to see what it was because I was intrigued but I'd actually had a drink so I couldn't, so I recorded it to stop people from thinking I'd imagined it!"
Mr McConnell described it as a "very eerie and kind of unnerving sort of sound" and revealed that he "kept checking back over the next few hours before I eventually went to bed and it was still going on".
He added: "You could only hear it from outside the house; the windows must have stopped it."
The writer said the recording on the short clip he put up on Twitter "isn't great" because "the noise was in reality much louder than it sounds on the video". "It also never varied in tone, didn't go up or down, it was constant and never changing whenever I listened to it," he said, adding: "It was like a horn."
The Co Down man also said it was "very difficult to tell which direction it was coming from... but if I had to guess I would say it was coming from the lough."
He added: "So it could have been a boat making a very loud noise with its horn but I don't know why it would last so long... it's a real curiosity."
Having turned to the internet for clues, Mr McConnell said there have been reports for years of a "hum" being heard in different places around the world.
Closer to home, he said there had been a case in east Belfast in May 2015 involving a drone that "was apparently traced to Palace Barracks" adding "but that was more metallic and whiny and high pitched".
Did you hear the strange hum in Strangford last weekend? Or do you know what it was? If so contact email@example.com