Autofocus, Manual Focus?

I'm thinking about getting the Nikon D3100, which is a DSLR that is capable of recording videos. I'm going to try to film beauty videos for Youtube which will include focusing on different products and its parts (color of it, name of it, etc.) I was wondering which focus is better for those videos: autofocus or manual focus?

I would use autofocus in these videos, but I saw in sample videos on Youtube that it creates a noise when pressing the shutter button to focus. Does the sound go away if you put the camera on a tripod or flat surface?

I'm not sure how manual focus will work in videos because I haven't tried it out yet. Explain?

If you are hand holding the camera in the video mode, you will want to use the auto-focus mode

If you have the camera mounted on a tripod like you would when using a real video camera, you can use either one. In the manual mode, you can use the lens to focus manually back and fourth between two subjects in the frame, just like you see on TV