Canon released a product advisory for the EF200mm f/2L IS USM & EF800mm f/5.6L IS USM lenses when used with an EOS 5D Mark III: It has been confirmed that when the Canon EOS 5D Mark III Digital SLR Camera is used with the EF200mm f/2L IS USM and some units of the EF800mm f/5.6L IS USM lenses […], the Lenses may emit an abnormal noise. This is not an issue of the 5D Mark III, it concerns the mentioned lenses. The units that are affected can be identified by the serial number. See the service note below to check if you have such an unit.