For a light, quiet sound:
* Sssshhhh
* Fffft
* Shhh-shhh
* Whisp
For a slightly heavier sound:
* Scritch-scratch
* Scribble-scribble
* Skritch-skritch
* Scratch-scratch
For a more dramatic effect:
* Skee-skee
* Screech
* Scrabble
Ultimately, the best onomatopoeia depends on the context and the specific sound you want to evoke.