Can the continuous autofocus (on video) be turned off on my Nikon D3100 so that it operates similar to the Canon 500D? This is mainly to remove the noise from the autofocus which I read in earlier posts.
Two easy ways.
Turn your lens to Manual focus. Nearly every lens has some kind of switch to toggle between modes of focus.
In camera, while in video mode.. press the "i" button, go down to where it says "AF-F" (the fourth option down on your menu, under the ISO setting) and change to "MF" or "AF-S"