The difference between する and している with onomatopoeias


For example, what would the difference between 膝{ひざ}がガクガクする or 膝{ひざ}がガクガクしている
be?

I believe the correct definition in Daijirin is
「恐怖{きょうふ}・疲労{ひろう}・緊張{きんちょう}などのために体{からだ}の一部{いちぶ}が小刻{こきざ}みに震{ふる}えるさま」, but that
doesn't help me see how the two differ.

or the difference between:

> 自分{じぶん}で出{だ}しておいてナンだけど、すごい **ドキドキする** 怖{こわ}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{こわ}いかも




Generally, `〜してる` indicates the state of that onomatopoeia, while `〜する` is
describing the stimulus that caused it.

Say you are watching an intense movie with a friend.

> ドキドキしてる = (My/your/his/their/our) heart is pounding
>
> ドキドキする = (This movie is) heart-pounding

Here is an example where the two can mean different things:

> 膝がガクガクしてるね = Your knees are shaking (but mine are not)
>
> 膝がガクガクするね = This is knee-shaking (for the both of us)

And the most critical distinction:

> あの人、ドキドキしてる = That person is excited
>
> あの人、ドキドキする = That person is exciting

