How many complain of rattling bikes in general? Loads!How many people complain about rattles from external cables?
With 11 speed cassettes, by producing narrower chains and spacing between sprockets on the cassette, the tolerances are tighter meaning more issues with keeping the drive train running smoothly. I'm not a luddite, but I sometimes wonder just how much of a benefit it is to the normal consumer when the industry produces ever more complicated parts which aren't backwards compatible.
The same argument could be made of 10 speed over 9 speed, 9 speed over 8 speed...
My 11 speed runs very well. In fact better than the 10 speed on my other bike. Unfair to compare as Ultegra vs 105, but it's a fact nonetheless.
What 11 speed are you running with internal cabling?