*Update*

I brought my 24-70 f/2.8L to Canon Service Center in Costa Mesa last Friday. I was told that the cable in the power diaphragm assembly over time would get loosened and stretched, the clicking noise is the result of this cable rubbing against other parts when zooming. The total cost is $244 including a new power diaphragm unit and 6-month warranty. I was also told that Canon will stop servicing this lens in January 2019.

Canon Service Center is a very nice place, they have the whole Canon lens lineup there on display, man it was my first time seeing the EF 11-24mm f/4L USM, that thing is humongous!