There are a few ways to look at this question. While on the river, Huck and Jim run into two con men, who call themselves the Duke and the Dauphin (King). These men pretend to be nobility, but "these liars warn't no kings nor dukes, at all, but just low-down humbugs and frauds." The Duke and King are the furthest thing from real nobility, but they play the part, "All I say is, kings is kings, and you got to make allowances. Take them all around, they're a mighty ornery lot. It's the way they're raised." (chapter 23)
However, the real noble characters in this book are Huck and Jim. Huck decides to help his friend even though he himself might be damned for doing so, "I was a-trembling, because I'd got to decide, forever, betwixt two things, and I knowed it. I studied a minute, sort of holding my breath, and then says to myself:All right, then, I'll GO to hell." (Chapter 30)
Huck helps his friend escape, not because its the right thing to do (he believes that it is wrong, based on what he has been taught) but because he just can't bring himself to turn in his friend. It is an innate instinct in Huck that he must do what he can for his friend, regardless of consequences. Jim, in turn, cares for Huck and helps him throughout their journey.
Another quote that I think illustrates nobility or noble behavior on Huck's behalf is when he watches the Duke and King get tarred and feathered. Even though the Duke and King were such terrible human beings and were mean to Huck, Huck didn't believe they deserved such a terrible death.
"Well, it made me sick to see it; and I was sorry for them poor pitiful rascals, it seemed like I couldn't ever feel any hardness against them any more in the world. It was a dreadful thing to see. Human beings can be awful cruel to one another." (chap 33)
Another instance of noble behavior/not-noble behavior you may want to look at is the lynch mob scene in chapter 22. In this scene a man named Boggs threatens a gentleman named Sherburn. Sherburn warns him to leave peaceably and Boggs doesn't, so Sherburn shoots him. The lynch mob forms to lynch Sherburn, but Sherburn talks them down:
"Your newspapers call you a brave people so much that you think you are braver than any other people-- whereas you're just as brave, and no braver. Why don't your juries hang murderers? Because they're afraid the man's friends will shoot them in the back, in the dark -- and it's just what they WOULD do."
He challenges the notion of "Southern justice" and chastises the crowd for being followers and getting caught up in a mob mentality.
Hope this gives you some things to think about!