One way Bradbury builds suspense is by mentioning the ventilator grille repeatedly. He says Montag thought about it and what lay hidden behind it without telling us what that hidden something is. He says that Montag looked at the grille. By mentioning something seemingly insignificant - the grille - in such a way, the readers knows this grille, or what lay hidden behind it, will take on importance at some point in the story. That's suspense because we don't know what is hidden there and when we do realize it's books, we don't know how important that revelation will be. Also, Bradbury mentions the Mechanical Hound in much the same way. We get a malevolent description of it and then ominous sounding statements about it and about something outside Montag's front door making a "scratching" sound. More mystery to the reader which builds suspense. The constant and increasing in frequency mention of the war planes and of war news also builds suspense in the story for the reader. The reader might dismiss the war planes flying overhead or the news on the radio at first, but the increase in frequency of mention in the story makes the reader aware that this, like the grille, will be important and that war is imminent.