Confirmed: The Canon EF 200mm f/2.0L IS USM Lens
' image stabilization system makes a modestly-disturbing churning sound when used on the Canon EOS 5D Mark III
in AI Servo AF mode. The same sound is made briefly when the shutter release button is released in One Shot AF mode.
I do not know if this is a problem or not. IS appears to be working properly and the images created during the strange sound are quite sharp. I'll let Canon know (others may have already done so).