This is because of the mechanism of the lens and camera; the mechanical noise of the internal parts operating may be recorded.
Particularly when panning the camera quickly up and down and from side to side, the noise may be easily recorded. Do not move the camera quickly, but hold it steady when you're recording movies.
NOTE: When you're using the camera, which is available with an optional external microphone (sold separately), try recording using the external microphone. Doing so can reduce the noise of the internal parts operating.