70-300L focus ring rattle
Happens with the 70-200 II as well if it is the same thing I had Canon check out.
My 70-200 II went in for IS repair... When I got it back I always check everything over very carefully. Noticed the focus ring had play in it... Like if I grab it and wiggle it on the axis of the lens itself, there is a tiny bit of play. I didn't know if it was like that before so Canon send me a prepaid label back to them to check out.
A lady from Canon NJ called me directly and said she went to a few techs and also compared with the rentals they had on site and was completely normal.