in my humble opinion, a comma would be useful. "Sara didn't finish because, just then a young man heard a noise" "because" is introducing the reason why Sara didn't finish. "just then" means "right at that moment". ie maybe the noise interrupted Sara and that is why she didn't finish.

It means that something just happened that did not allow her to finish, its not typically used in this manner however. Better example would be, "She was drinking coffee but stopped suddenly because just then she got a text saying that her brother was in the hospital"