what profound question does Daisy whisper into Gatsby's ear

In chapter 5, she does whisper in Gatsby's ear at the end but we don't find out.

"As I watched him he adjusted himself a little, visibly. His hand took hold of hers, and as she said something low in his ear he turned toward her with a rush of emotion. I think that voice held him most, with its fluctuating, feverish warmth, because it couldn’t be over-dreamed — that voice was a deathless song".