Well, I guess it depends on the situation, but mostly it's "うーん" or "ううん".
If your Japanese friend is thinking about the answer or how to react, it's more like "うーん、、、", in this case, you may also use "えっと" in Japanese.
On the other hand, if they were trying to say NO, in Japanese, it's "ううん↑".
I guess manga lovers have more expression. Let's see if other answers come up later.