> Could our noble Mr Knightley be (gasp) jealous of young interloper Mr Frank Churchill?

In the words of Homer Simpson: "Well, duh! He's breathtaking."

More seriously: Mr Knightley has rational reasons for disliking Frank Churchill (Frank's neglect to visit his father, the fact that he went to London to get his hair cut...). However, his dislike is uncharacteristically strong; he doesn't accept any of Emma's arguments in favor of Frank in Ch18, and his opinion so far never seems to improve. So it's not completely out of line to suspect that he's not absolutely candid.