Just to add mine. Also when AF fails, shifting or attempting to focus seems to produce a high frequency sound from the lens. Based on my engineering experiences, seems like a contact problem which causes the lens motor to be not properly powered to have enough torque.
890101XXXX on 5D3
Yah, the sound on mine was like very short bursts of the motor, but nothing happening.
In your opinion, do you think there's the possibility that the mount itself is stiff when new, creating improper contact? If so, this would actually explain why the disappearance of this issue correlates with the mounting of this lens 'wearing in' and no longer feeling stiff....
I dunno, pure speculation, but food for thought.