On Saturday, U.S. astronaut Barry Wilmore reported a repetitive knocking sound coming from the Boeing Starliner to NASA’s ground crew. The interaction, captured by a NASA Space Flight forum member and later spotted by Ars Technica, includes a recording where Wilmore asks the Houston team to listen to the noise, which he described as coming from the craft’s speaker. The sound, resembling a “sonar ping,” could be heard in the background.

While the cause remains unclear, some speculate it could be a specific type of feedback loop. Despite the eerie noise, neither Wilmore nor the NASA ground team appeared overly concerned. Boeing’s Starliner is scheduled to undock on September 6th for an automated return to Earth, with Wilmore and fellow astronaut Sunita Williams staying on the ISS until early next year.