Gitzo carbon fiber creak?
First, I should mention that the noise went away shortly after posting that (ironic) and now only consistantly occurs at the very bottom of the post (meaning when it's fully raised) and only when it's oriented at a 90 degree angle.
After hours of trial and error, I've discovered the source of the noise. It's not the centerpost itself, but rather the friction between the post and the magnesium clamp bearing down on it. Gitzo designed it so the metal part of the bottom plug (where the counter-weight hook is) would perfectly clear the magnesium portion of the tripod that joins the legs. However, they put a thin rubber O-ring around that plug to keep it from damaging the finish if you were to carelessly pull the post up. Because the tolerances were so exact for the metal plug, it can't fit with that O-ring. So when you put the centerpost in its highest position, that O-ring cocks it by about 1/16". Now when you tighten the clamp, the rubber yields and the post uncocks itself while under the pressure of the clamp, causing a chirp/creak of friction. From what I can tell, this is inherent to the moveable-post design the Explorer series features.
It's hardly noticeable, I only made such a big deal out of it because I errantly feared for the brittle carbon fiber.