I too had this problem, so here's my story with the lens.
14-01-2014: I purchase this lens from a local store in Denmark, and got it home - Serial 042. Immediately I notice the clicking noise when zooming. The description of the problem is very similar to others. Slight 'ticking' noise when zooming from 70mm to 24mm. The zoom ring also feels quite tight, especially going from 70-24.
Despite this, I ran it through Reikan Focal and did an extensive AF test. I found it had quite erratic autofocus. While it certainly did achieve sharp results, the focus was all over the place, despite having run it through AFMA test and set it to its best (Required +6 on wide and +8 on tele).
06-02-2014: I take it back to the store, and luckily, they had a 24-70 on display. It did not have the ticking noise, and the zoom ring was also a lot smoother. They also had 3 other new lenses in the back, and took all of 'em out. 2 of them, serial 032 and 042 had similar problem as the one I purchased. The ticking was quite pronounced on the 032 serial, while the 042 was harder to hear, but it was there. The 3rd was a 096, and had the old lens cap - it was perfect. No noise, smooth zoom ring, just like the one on display, which also had serial 096.
I ended up exchanging it for the new 096 serial, and took it home. Did some AFMA test, it required +3 wide, and +4 tele. The AF results were similar to my other primes/zooms, with much more steady results. Sharpness was the same as the first one at its best.
I also noticed that the Aperture sharpness was very different on the 2 lenses. The 'ticking' lens started low in sharpness at 2.8, and steadily increased until 4.0 and slowly dropped until 10.0 where it dropped fast. The new lens was sharpest at 2.8, and dropped very very slowly until 10.0, where it started dropping fast like the other.
All tests were done in the same room, with the same light in the evening at 8,8 EV. Tripod at same spot, both at 24mm zoom at 1.2meters.
Hope this info helps someone.
I also want to thank you all for a great community, with lots of great info. I've read a lot on this forum, but this is my first post. I figure I could finally contribute with something