I previously reported that my March 2013 copy of the 24-70mm (serial number beginning 032) does not make a clicking noise when zooming. I've been using my lens as usual for about 6 months, but all these additional reports got me checking my lens again.
After many tests in a completely quiet room with my ears right next to the lens, I report that I can hear a very faint single click sound as the lens barrel expands past the 50mm mark (like a tiny spring or the inner cams moving into place). This sound is really faint -even the zoom ring moving is louder-, and it takes moving the zoom ring very slowly to even catch it. This is not what you guys are talking about, right?
This is exactly my experience, PT!
After purchasing the lens from B&H and discovering this thread, I was hyper-sensitive when I opened the box and tested the lens. I can only hear a faint sound when moving the zoom ring really slowly.
My build is 031 (March 2013) with the center-pinch cap.