Edited : I honestly speaking did not know what "onomatopoeias" means. Sorry.
However, given that the term refers to the same repetitive sounds, such as woof-woof ( bark-bark ) for a dog, does it make any difference...........?????
What would the difference between 膝がガクガクする or 膝がガクガクしている be?
So, as I mentioned below, the former denotes only the motion, meaning in English
"My knees shakes? quivers? ( in English what term is proper? Would you kindly teach me. )"
and the latter denotes as I mentioned below, the "continuation of the motion" probably equivalent with in English "be + ing"
"My knees keep shaking, quivering...".
I believe the correct definition in Daijirin is 恐怖・疲労・緊張などのために体の一部が小刻みに震えるさま
regarding this, it depends upon the very situation caused by your action for what reason your knees shakes, or keep shaking....this is only what I can say.
or the difference between
自分で出しておいてナンだけど すごいドキドキする 怖いかも
自分で出しておいてナンだけど すごいドキドキしている 怖いかも
regarding these, actually, I do not understand. Especially this
自分で出しておいてナンだけど, meaning, "Though I put? ( pull out? ) this out of ( from? )
*****". Only I can say is please be more specific......
I am sorry after my repetitive thinking, I am not able to give you any alternative interpretation.
Simply put, it is the difference between "the motion ( or action ) itself" and "the continuation of the motion ( or action, )".
For example : My sister gets married this year 私の妹は今年結婚する。( only denoting the
action itself )
and the latter example is
My sister is already married: 私の妹は結婚しています ( denoting the "continuation" or rather here the "condition" ( continuation inferred so )).
If you are satisfied here, please forget the following supplementary. ( I would like to go
into further discussion for anyone interested here )
Japanese is controlled particularly by auxiliaries or particles, and verbs.
I would like to divide each する and している into the above pieces.
する = verb. in English "do" ( and in your English question, the verb is used as a
"conclusive form" ( verbs vary according to what comes after ))
している = し/て/いる。
し = varied form of する because as I mentioned above, verbs vary the form according to what comes after.
て = conjunctive particle : only denoting nothing but put here for something coming after this.
いる = auxiliary "verb" : in English equivalent with "be" denoting the continuation, condition, etc
Have a nice day.