That's your hand sliding down the strings. Whenever two things that aren't perfectly smooth rub against one another, they cause slight (usually high pitched) vibrations. Think about running your nails down a chalkboard, or peeling out tires, or dragging a cardboard box across the floor. It's unavoidable, but can be reduced by using a lubricant such as "Finger Ease", or using Elixir strings (they're coated strings). String talk is more commonly heard on acoustics because of the more sensitive pickups.