Hi Mats,
It sounds like you want it to ‘boil’. Boiling makes otherwise stationary objects look like they’re moving and can make animation more dynamic, but it’s essentially an artefact. The easiest wasy to achieve it is to draw two or three versions of the same picture and then play them one after the other (2 – 3 frames each, or you can experiment to see what works best). Because your pen will be slightly off target each time, the lines that make up the drawing will appear to move, or ‘boil’.
To achieve this in AE, import your 2 or 3 frames as an image sequence and then use the interpret footage dialogue box (I think this is where the option is) to loop the sequence as many times as necessary (I usually just give it a very high number). Then when you drag the layer out in the timeline, it will continually boil.
Hope this is what you mean.
Simon Bonner
youtube.com/simonsaysFX