Starliner is scheduled to undock from the International Space Station and make its return trip to Earth uncrewed in just a matter of days, but it apparently still has a few new mysteries left in it to throw at the team before it departs. On Saturday, astronaut Butch Wilmore alerted NASA’s Mission Control about an unexplained “strange noise” coming from a speaker in the spacecraft, which you can hear in an audio clip of the conversation shared on a NASASpaceflight forum by meteorologist Rob Dale (spotted by Ars Technica). It starts at around the 45-second mark, ringing out on a steady beat. “I don’t know what’s making it,” Wilmore said.

After confirming they could hear the sound too, once Wilmore brought his mic over to the speaker, the flight controller in Houston said, “It was kind of like a pulsing noise, almost like a sonar ping.” Wilmore then lets it play for about 20 seconds more before wrapping up the call. “Just to make sure I’m on the same page, this is emanating from the speaker in Starliner,” Mission Control asked, “you don’t notice anything else, any other noises, any weird configs in there?” The astronaut notes that everything else seems normal.

It’s still unclear what caused the sound. The Boeing spacecraft has been docked with the ISS since early June, and engineers have since had their hands full trying to get to the bottom of the issues that arose during its first crewed flight. When Starliner finally heads back to Earth on September 6, it’ll be leaving its crew — Wilmore and NASA astronaut Suni Williams — behind on the ISS, where they’ll continue to work for the next few months while they wait for a ride home from SpaceX in February 2025.