Was out in the cold with the 50D and 100-400 L today, when suddenly I heard a fast, repetitive whirring sound; almost as if the AF motor was stuttering. I was just holding the camera in my hand at the time (i.e. not shooting), and looked down the lens to see the elements inside 'twitching', as though the camera were trying to continually shift focus.
This strange performance repeated a few times during the day, each time while the camera was idle in my hand. In a later episode I also noticed that, while the AF 'twitch' was going on, the aperture and shutter values displayed on the top screen would 'twitch' too, very quickly alternating between 1/400 and 1/500th and f/8 and f/9. The incidents would only last a few seconds and performance seemed unaffected.
Additionally, every time I looked through the viewfinder and held the shutter button half-way to focus, the image would 'jump' before settling down. This happened whether the IS was on or off.
Anyone had similar experiences, and should I be concerned?