Canon has issued an advisory on the issue http://www.usa.canon.com/cusa/support/consumer/eos_slr_camera_systems/eos_digital_slr_cameras/eos_5d_mark_iii?pageKeyCode=prdAdvDetail&docId=0901e02480532d6b
To Users of the EF200mm f/2L IS USM and EF800mm f/5.6L IS USM Lenses with the Canon EOS 5D Mark III Digital SLR Camera.
Thank you for using Canon products.
We have confirmed that when the EF200mm f/2L IS USM and some units of the EF800mm f/5.6L IS USM lenses are used with the Canon EOS 5D Mark III Digital SLR Camera, the Lenses may emit an abnormal noise.
The Canon EOS 5D Mark III Digital SLR Camera does not need to be repaired as a result of this phenomenon.
The support measures for this phenomenon have been decided and they are as described below.
If either of the Lenses is attached to the camera with the Image Stabilizer switch on the Lenses set to ON, the Image Stabilizer will synchronize with the autofocusing (AF) function and an abnormal noise may occur. This abnormal noise does not affect captured images.
This phenomenon only occurs when the camera is used with either of the Lenses, and does not occur when the camera is used with other lenses.
EF200mm f/2L IS USM Lens
EF800mm f/5.6L IS USM Lens
*This phenomenon does not occur with cameras other than the EOS 5D Mark III.
*If the fifth and sixth digit of the six-digit alphanumeric number that is imprinted on the lens mount of the above EF lenses is of any of the following numbers, the lens is affected.
EF200mm f/2L IS USM： If the fifth and sixth digit is 00, 01, or 02.
EF800mm f/5.6L IS USM：If the fifth and sixth digit is 00, 01, 02, 03, 04, 05, or 06.
The Lenses affected by this phenomenon will be inspected and repaired free of charge beginning May 21, 2012. If you own one of the affected Lenses, please contact our Customer Support Center.
This information is for residents of the United States and Puerto Rico only. If you do not reside in the USA or Puerto Rico, please contact the Canon Customer Support Center in your region.