The humanlike noises began before takeoff and persisted in the air, especially early in the flight and during service, Collins told The Washington Post. In the video, the sounds range from groans and grumbles to whoops.

“I swear it’s a prank,” a flight attendant can be heard telling Collins in the video.

At one point, a flight attendant comes over the intercom to apologize for the “extremely irritating sound” and says the pilots are working to mitigate the problem. As soon as she finishes speaking, the sounds reemerge, resembling a “ho!” and a guttural moan.

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Collins said he walked the aisle looking for the source of the noises, expecting it would be obvious in the confined space of an aircraft cabin.

“I’m Nancy Drewing my way looking for the person who looks thoroughly amused by themselves,” he said. “And, of course, I didn’t see anything.”

The Airbus A321 landed safely in Dallas. Collins said he found the incident amusing and that the flight crew reassured passengers there was no safety concern.

“This was fully just a very modern form of immersive in-flight entertainment,” he said.

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After Collins posted the video last week, the mystery deepened as passengers on other American flights in recent months reported the same array of noises.

“It wasn’t the whole flight, but periodically weird phrases and sounds. Then a huge ‘oh yeah’ when we landed. We thought the pilot left his mic open,” journalist Doug Boehner tweeted about his recent Orlando to Dallas flight.

Tech executive Brad Allen wrote that he and his wife experienced the noises on an American flight in July.

“To be clear, it was just sounds like the moans and groans of someone in extreme pain,” Allen wrote. “The crew said that it had happened before, and had no explanation.”

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Another user said the noises occurred on her Aug. 5 flight from New York to Los Angeles, and aviation commentator @xJonNYC shared an account from a Santa Ana to Dallas flight on Sept. 18.

The incidents sparked amateur sleuthing online, with theories ranging from a prank like a passenger dragging the crew microphone into the lavatory to the more nefarious suggestion of a hack into in-flight systems.

American Airlines spokesperson Sarah Jantz said in a statement that intercom systems on the airline’s planes are hardwired without any external access or WiFi component.

“Following the initial report, our maintenance team thoroughly inspected the aircraft and the PA system and determined the sounds were caused by a mechanical issue with the PA amplifier, which raises the volume of the PA system when the engines are running,” she said. The first report the airline received was the Sept. 18 Santa Ana-Dallas flight, according to Jantz.

“Our team is reviewing the additional reports,” she added.

Collins said if the sounds were the result of a technical problem, then “the ghost in the machine has a hilarious sense of humor.”

“The comedic timing of the sounds seemed entirely too planned to be able to just be some technical glitch,” he said. “But honestly, I have no idea.”