What scratches on Montag and Millie’s front door while they are reading? How does Montag react to it?

The Mechanical Hound scratches and sniffs around the Montag house while Montag reads aloud to Millie. At first, Montag freezes and tells Millie to be quiet in what appears to be paralyzing fear. But that moment is soon followed by a grim resolve on Montag's part to resume reading once he thinks the Hound has moved on.

"Let's get back to work," said Montag quietly. (page 72)

Montag attempts to resume reading, but he has...

The Mechanical Hound scratches and sniffs around the Montag house while Montag reads aloud to Millie. At first, Montag freezes and tells Millie to be quiet in what appears to be paralyzing fear. But that moment is soon followed by a grim resolve on Montag's part to resume reading once he thinks the Hound has moved on.

"Let's get back to work," said Montag quietly. (page 72)

Montag attempts to resume reading, but he has lost Millie's attention and interest. Montag begins to think about the occurrences he observes in the world around him, such as the constant roar of jet bombers heard overhead, and the fact that: "We've started and won two atomic wars since 1990!" (page 73)

He begins to realize that he takes these major events for granted and wonders why. When the telephone rings, Mildred is quick to snatch the phone and answer, becoming instantly distracted, as usual, by a shallow conversation about a television program. Bradbury cleverly provides the answer to Montag's question with the intruding call and Millie's Pavlovian response to it.

Montag tries to continue reading and ignore Millie's chatter but notices that he is easily distracted as well. His mind is challenged to concentrate and he struggles to derive meaning from the words he reads, despite all his curiosity and intent.

"Poor Millie, he thought.  Poor Montag, it's mud to you too. But where do you get help, where do you find a teacher this late?" (page 74)

Montag turns a corner in the novel at this point, committing to a path that he cannot turn back from. He recalls a man he met in a park whom he came upon in possession of a book. In a foreshadowing of his later revolt against the system, Montag declined to arrest the man (Professor Faber) at the time. Montag reacts to the threatening presence of the Mechanical Hound at the door by deciding to contact Faber, rejecting the Hound and the system of repression of thought (or lack of thought) it enforces.  He resolves to continue on the path of self-realization.