Scratching generally happens on the hard cover of a book, a plastic bottle, a table, or another hard surface where it can be most useful. The sound can either be very gentle or a bit grating depending on how hard the person is scratching on the surface. Either one can produce the desired effect, though.
Blowing, particularly into your ear, brings both the sensation of the feeling of cool air brushing against your skin and the sound that the other person’s mouth makes as they accomplish this task. Moving from ear to ear helps really bring this trigger to the forefront for most people.