A mysterious, pulsating sound heard by one of two stuck-in-space astronauts coming from Boeing's troubled Starliner has been identified.

Butch Wilmore is one of two NASA astronauts whose return to Earth is delayed due to Starliner's technical difficulties.

Wilmore recently heard a strange noise from a speaker in Starliner while stuck aboard the International Space Station, NASA wrote on X on Monday.

"The speaker feedback Wilmore reported has no technical impact to the crew, Starliner, or station operations, including Starliner’s uncrewed undocking from the station no earlier than Friday, Sept. 6," the space agency wrote.

NASA attributed the noise to an "audio configuration between the space station and Starliner."

"The space station audio system is complex, allowing multiple spacecraft and modules to be interconnected, and it is common to experience noise and feedback," NASA wrote.

On Sunday, retired Canadian astronaut Chris Hadfield raised concerns about the noise.

"There are several noises I'd prefer not to hear inside my spaceship, including this one that @Boeing Starliner is now making," he wrote in a post on X, including a video with the pulsing sounds.

Fellow stuck astronaut Suni Williams and Wilmore were Boeing's first crew and were only supposed to be in space for a week in June, but their return to Earth was pushed back to February due to technical problems with the space capsule. NASA decided it was too risky for them to fly home on the Boeing Starliner because it was experiencing thruster troubles and helium leaks. NASA cut two astronauts Friday from the next mission on a SpaceX rocket in order to make room for Williams and Wilmore on the return trip next year.

Their empty Starliner capsule will depart from the space station and land in the New Mexico desert as early as Friday.

With files from The Associated Press