Musar zu Bereschit 24:40

Having explained all this, we can now understand the difference between Genesis 6,1 את האלוקים התהלך נח, "Noach walked with the Lord," and Genesis 24,40 when Abraham describes himself in these words: ה' אשר התהלכתי לפניו, "G–d before Whom I walked" Noach needed G–d to "hold his hand," so to speak, whereas Abraham was self-propelled, took the initiative. Noach was afraid to mix with the corrupt society he lived in and isolated himself with only G–d as his companion because he was afraid of the possible influence on him of contemporary society. Abraham was not only confident that he would not succumb to the corrupt society around him, but he tried to lead his fellow-men back to the path of monotheism and a life of good deeds. I have elaborated on this elswewhere. This is the plain meaning of those verses.

Noach then was righteous in terms of his generation since he responded to inspiration from "above." G–d "encouraged" Noach during the one hundred and twenty years prior to the deluge. He sent him on a mission to warn his contemporaries to repent of their wicked ways. The Torah however, records only מקצת שבחו בפניו, וכלו שלא בפניו. The usual meaning of this concept is that one does not reveal all of a person's merits while he is alive, whereas when eulogizing such a person one must reveal all his merits. In this instance the meaning of שלא בפינו is that Noach did not initiate his role as the conscience of mankind, but he was prompted to do so by G–d.