Bruno accidentally lets slip that he knows there are children living in Auschwitz, briefly dumbfounding his father and sister. Boyne uses an oxymoron to convey the tension at the table in the aftermath of Bruno’s revelation:
A silence followed this remark, but it wasn't like a normal silence where it just happens that no one is talking. It was like a silence that was very noisy. Father and Gretel stared at him and he blinked in surprise.
The oxymoron "a silence that was very noisy" refers to the intense atmosphere of surprise and shock following Bruno’s accidental revelation. While no words are exchanged in this passage, the phrase illustrates how Bruno’s slip of the tongue conveys far more than he actually says. Instead of responding with words, his father and Gretel respond with silence. The absence of sound becomes overwhelming, unlike “a normal silence where [...] no one is talking.” The way that Bruno "blink[s] in surprise" further underlines this tension, showing how the moment is filled with the unvoiced significance of Bruno’s words. The silence is “very noisy” because everyone experiencing it is having notable realizations for themselves: Bruno realizes he may have given away his friendship with Shmuel, and Gretel and their father are immediately concerned about how much Bruno knows about life and death in Auschwitz.