US astronaut Barry Wilmore called NASA ground crew on Saturday, asking for help with a repetitive knocking sound that was coming from the Boeing Starliner craft. The interaction was captured by a NASA Space Flight forum member, who included a recording of it in a post that was spotted by Ars Technica.

In the recording, Wilmore asks NASA crew in Houston to configure their call so that he could show them the noise, which he says is coming from the speaker inside Starliner. Then, a repetitive clanging sound with slight there’s-something-on-the-wing vibes can be heard. The Earthside crew member describes it as sounding “almost like a sonar ping.”

The mysterious sound could very well have a very mundane explanation, like the very specific type of feedback loop one person on Reddit compared it to. At the very least, neither Wilmore nor the NASA ground team member he spoke with seemed particularly concerned (even if it does sort of resemble the unnerving Ganado theme from Resident Evil 4).

Boeing’s Starliner is currently scheduled to undock on September 6th for an automated, uncrewed return to Earth, leaving its original intended passengers, Wilmore and fellow astronaut Sunita Williams, behind on the ISS until early next year.