Let's see. This is full frame lens? STM is quiet during focusing, presumably so that the camera can focus while taking videos. (Then why are the 24 f/2.8 IS and 28 f/2.8 IS lenses USM, not STM? Because they're IS lenses perhaps?) The 5D3 can't focus while taking videos, but there is a firmware upgrade due for the 5D3, really soon now, with the rumor that features will be added. Dare we hope?
And while you're at it, Canon, make exposure compensation and automatic exposure bracketing functional in the 5D3 in manual mode with auto ISO. The present situation is inexcusable.