The references to the ventilater grille are instances of foreshadowing, a technique used by authors to give slight clues to future events that are going to occur in the story. If you pay close attention to the clues given, you can use them to predict what is going to happen later on in the story, or predict why their inclusion in the story is significant. Looking at these quotes, several clues are given, that if we piece together, provide us with rather ironic information about Montag, a burner of books. The first reference comes after his first meeting with Clarisse. She asked him if he was happy. He thought it was a strange question, and defensively asserted that he was. But then his thoughts go immediately to "something [that] lay hidden behind the grille." He has hidden something there--something that reminds him that he really isn't happy burning books and living his life as it is.
Later, the hound growls at him, and he worries that someone at the firestation had told it about what was hidden behind the grill. This means that what is there is something that a fireman would be called in to take care of--this seems to suggest a book. This is later confirmed as Montag thinks of the grille as he is thinking of forbidden books. All of these references suggest Montag is not happy with his station, and that he has a burning desire to find out what the real deal behind books is. It foreshadows a possible rebellion and quest on his part, which is, essentially, what the book is about. I hope that helped; good luck!