In the JP version, the text was just a squiggle that you have to look hard to read it, but it's あ〜〜〜る ... which, when you read the final novel, you realized that she said あいしている which is "I love you".
For the scanlated version, I tried to write "I love you" in cursive and very squiggly just to match the JP version.
I think this was the reason why Kaori's face lit up like that after hearing Shizuku's whisper, and it's really author's intention for us to find out what Shizuku said to her ... from the novel in the end.