We've been able to isolate the sound to the camera body itself, so it's completely independent of the lens and other accessories. We also happen to be testing a variety of new Sony products this week in San Diego, and had Sony reps on hand to acknowledge and confirm the issue, which seems to plague every NEX-5N that we've tried. This is still very fresh, so Sony doesn't have a fix or even a statement to issue, but because 1080/60p video is one of this camera's headline features, we're going to recommend holding off on your purchase until the company can confirm what's causing the issue. Jump past the break to hear it for yourself in our demo video.
Update: To clarify our experience, we've only tested four cameras, all of which were likely manufactured at the same time. All four cameras presented the same clicking sound, but Sony is still investigating the issue, and has by no means implied yet that all NEX-5Ns are affected. We will of course share any updates as we receive them from Sony.
[Thanks to everyone who sent this in]