"Experts tend to think that the sound came not from the outside but from the inside of the station, namely from the instrument panel of the Zvezda module," he said. "But the conclusion is not final, as Kaleri was in headphones at the time."
Engineer Kaleri heard the sound that resembled a slap at 11:00 Moscow time on Friday. He reported it to specialists back on earth, who are analyzing possible causes for the sound. All ISS systems are working normally and the equipment has not registered any malfunction, say specialists of the Flight Control Centre.
A ranking official of the center had told RIA Novosti that "the slap which Kaleri heard did not repeat and was not at all like the dub of a drum, as some media hastened to report."
In November 2003, Kaleri's colleague, NASA astronaut Michael Foale heard a sound which he described as a flapping sheet of metal against the ISS exterior. Experts could not agree on its origin then. According to a version, it came from a ventilator.