It is pretty upsetting that Canon allowed such QC issues on any lens much less a premium lens for so long.
I am on my 4th copy. This one is optically fine, no click, no squeak, but a single very tiny bubble in the front element. I can't find any impact on IQ, and may keep this copy, largely because I am more concerned about what problem the next lens I get has given everything I have read in this thread.
BTW, my tally: first copy: 032-loud clicking, second copy: 032-loud clicking, third copy: 062-AFMA/softness issues at 70 mm, and 4th copy: bubble in front element.
So, I'll either keep this copy, send it in to Canon under warranty, or I will return it for a refund and keep my 24-105 for at least another year.