That's the sound made by the "Multiple Kill Vehicle," a frightening but fascinatingly cool hovering robot meant to shoot down enemy ballistic missiles.
Video of a Dec. 2 flight test conducted at Edwards Air Force Base in California by defense contractor Lockheed Martin has made it onto the Web, and it looks like something out of the "Terminator" movies.
Rival defense contractor Raytheon is also working on its own multiple-kill-vehicle program.
Inside a large steel cage, Lockheed's MKV lifts off the ground, moves left and moves right, rapidly firing all the while as flames shoot out of its bottom and sides.
The plan is to mount one or more MKVs onto carrier missiles, which would launch into space to engage enemy nuclear-tipped ballistic missiles at the apogees, or peaks, of their trajectory arcs.
Once in space, the MKVs would break off from the carrier vehicles, then use highly accurate targeting computers to shoot big bullets — in military speak, "kinetic interceptors" — to destroy the enemy warheads before they drop back down to Earth.
None of this description does any justice to the video. You have to see it for yourself.