Why does the Mechanical Hound growl at Montag, and what does this symbolize?
When Montag greets the Mechanical Hound, it responds by growling at him. This is an unusual response, given that Montag is a fireman and that the Mechanical Hound is a tool of his trade. Moreover, the Hound is designed to obey, as Captain Beatty comments:
It doesn't like or dislike. It just functions.
As noted in the previous answer, this growl represents Montag's growing sense of internal rebellion and also foreshadows future events in the novel. But the growling is also symbolic of the dangers of censorship—a central theme in the novel. In Bradbury's world, for example, the Hound is used to literally sniff out people who read books. As we see later in the novel, the punishment meted out by the Hound for such a crime is swift and violent. To desensitise people to such violence, the government use the Hound's performance as a form of entertainment by broadcasting it on live television. In this way, the Hound's growl represents the totalitarianism of the government in Fahrenheit 451 and acts as a reminder to the reader that censorship is one of the most dangerous forms of control.
The Mechanical Hound is a machine that is meant to track down and kill people who have gone against society (by having books or something like that). That is its only role.
We don't really know why the Hound growls at him -- it shouldn't do that unless it's programmed to. But when it growls at Montag, that shows us that Montag is becoming something of a dissident. He has been keeping books, but we don't know that yet.
The Hound growling at Montag is a foreshadowing of what is to come. It is a clue to us that he is doing something that would make the authorities unhappy with him.