In the 7. episode of season 2 we see detective Clausen looking at a photo which I assume was from his youth and standing next to him is his missing brother Alexander. When I saw this scene I thought immediately that the brother was Noah. I did check that scene a few more times and I think that there is a strong resemblance to young Noah but I can't be sure.
Was there ever any information from the producers about that scene or about Alexander Köhler in fact being Noah?
Based on what the show has told us, Clausen's brother is the person
that Alexander Kohler (Boris Niewald in reality) presumably killed
before meeting Regina and took over his identity.
Young Noah was called Hanno Tauber at birth and was alive in 1921, as
was his sister Agnes Nielsen, who later killed him. Also, in 1921, he wasn't a time traveler yet. So he was killed at a different time, not in 1986 as probably was Clausen's brother.
All these facts lead to the conclusion, that Noah can't be Clausen's brother.