I never really thought about this. I've learnt something new today.

There is no general consensus as to what onomatopoeia should be used to represent this sound.
I used to use "tchk tchk tchk tchk tchk," but I now prefer "krrk krrk krrk krrk krrk." I've seen Japanese fans use カキココケ ("ka-ki-ko-ko-ke") which works pretty well IMO.