Nikon D300 User Manual page 718

Continuous-servo AF Mode (C)
Similar to Single-servo AF, you must be sensitive to the
status of your subject to get the best results. Using
Continuous-servo AF is slightly more complex.
Subject is not moving: When the subject is standing still,
Continuous-servo AF acts a lot like Single-servo AF with the
exception that the focus never locks. If your camera moves,
you may hear your lens chattering a little as the auto focus
motor makes small adjustments in the focus position. Since
focus never locks in this mode, you'll need to be careful that
you don't accidentally move the AF point off of the subject
because it may focus on something in the background instead.
Subject is moving across the Viewfinder: If your subject
moves from left to right, right to left, or up and down in the
Viewfinder, you'll need to keep your AF point on the subject
when you are using Single-point AF area. If you are using
Dynamic-area AF or Auto-area AF, your camera will have the
ability to track the subject across a few or all of the 51 AF
points. We'll cover this in more detail in the upcoming
section called AF-Area Modes in Detail.
Subject is moving toward or away from the camera: If your
subject is coming toward you, another automatic function of
the camera kicks in. It is called predictive focus tracking, and
it figures out how far the subject will move before the shutter
fires. Once you've pressed the Shutter-release button all the
way down, predictive focus tracking moves the lens elements
slightly to correspond to where the subject should be when
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