NASA astronauts at the International Space Station reported Saturday hearing a “strange sound” coming from the Boeing Starliner rocket just days before the spacecraft is scheduled to depart the station and return on autopilot to Earth.

Butch Wilmore radioed Mission Control in Houston, Texas, to ask about the noise.

Wilmore held up his phone so Mission Control could hear what he meant. Wilmore’s device emits a pulsating noise at regular intervals.

Mission Control, who hadn’t heard it before, says “Butch,” after hearing the sound for the first time. It was almost like a sonar sound, pulsating.

Wilmore, who played the sound again, told Mission Control: “I’ll play it one more and let you scratch your heads to see if you guys can figure out what is going on.”

Mission Control informs Wilmore that the recording is being passed on and they will let him know what’s found.

Wilmore clarifies the source of the sound as being the Starliner’s speaker.

Ars Technica reported the strange sound first, citing a recording captured by Michigan-based weatherman Rob Dale.

Starliner will attempt to land in New Mexico’s desert on autopilot after it docks from the ISS.

NASA decided that it was not safe to return Wilmore and Suni until February. The mission was originally scheduled for a one-week trip to be undertaken in early June. However, problems have plagued the mission due to thruster failures as well as helium leaks.

Boeing hoped that the first crew trip of Starliner would revive its troubled spacecraft project after years’ worth of delays and rising costs. Starliner’s safety was based on recent thruster testing in space and on Earth.