...checked out a rental copy of the 24-70 II and it has the same clicking noise, although it's a lot quiter after heavy use.
So, the internal components that are rubbing together to produce the clicking noise wear down with use, reducing the noise? I'm not sure that I'd take that as a good sign.
It is a very good sign.
A very good sign that you now have small fine metal filings inside of your lens.