This quotation is taken from towards the beginning of the book, when Claudia and Jamie have decided to go into the city, and seek to achieve their goal through staying on the school bus after it has done its rounds and stowing away, receiving a free ride into the city. When they come to enact their plan, however, it is clear that Claudia is very nervous and fearful of being discovered. The full quotation is listed below:
Otherwise, Claudia would have worried for fear the driver could hear her heart, for it sounded to her like the electric percolator brewing the morning’s coffee. She didn’t like keeping her head down so long. Perspiration was causing her cheek to stick to the plastic seat; she was convinced that she would develop a medium-serious skin disease within five minutes after she got off the bus.
What saves Claudia, to her mind, is the noise that the bus is making, as it prevents the driver from hearing her heart, so loudly does it sound to her own ears. The comparison of the sound of her heart to the coffee percolator is a comparison that exaggerates the sound of her own heartbeat and shows just how nervous and excited Claudia is. The coffee percolator is a machine that produces a lot of noise, and so this is an apt choice to convey Claudia's emotions at this critical time as she tries to achieve her goal of running away and reaching the city. What she fears is that she and Jamie will be discovered on the bus, and that their plan will come to naught.