Back in 2012 Canon released the 5D Mark III and had some issues when used with select Canon EF 800mm f/5.6L IS USM and Canon EF 200mm f/2L IS USM lenses.
The issue being an abnormal operating noise with the IS and USM mechanism when AF is engaged and IS was ON.
I was one the few affected by this incompatibility that would require the lens to be shipped back to CPS, disassmbled and a part swapped out.
Not wanting to mess with a perfectly working lens I decided to skip the 5D Mark III.
Now, with the 5Ds R I can say my 800 operates as quietly as if it was attached to a 7D Mark II.
On a side note for those who received their bodies within the past week what is the last digits of your serial number? Mine is less than 50.