Is NASA watching another catastrophe in the making?: "The two-man crew aboard the international space station reported a strange external noise early Friday—the same unexplained metallic sound that startled them just before Thanksgiving.

"The source of the noise outside the station's Russian crew module, which resembled the flexing of a metal sheet, has not been identified."

"Engineers speculated the first incident was caused by orbital debris striking the outpost, contraction and expansion as the station sailed between sunlight and darkness, or a loose fixture in the humidity control system.

"A February spacewalk in which the two men planned an inspection of the sound source was cut short by a spacesuit problem. Foale and Kaleri are due back on Earth on April 29, ending a 6 1/2-month mission."