The metaphor of whispering is commonly used as foreshadowing throughout the story. Whispering is used to hint at dangers or trouble yet to come. The revolution is always in the background, and will eventually engulf all of the characters in much more than a whisper. Much like a wave, its influence starts out small and then gathers momentum—when a whisper becomes a roar.
When the Manette family settles down outside, the trees whisper. This is a message of trouble to come.
Mysterious backs and ends of houses peeped at them as they talked, and the plane-tree whispered to them in its own way above their heads. (book 2, ch 6, p. 65)
Another example is more literal. Throughout the revolution, everyone talks in whispers.
“All the village,” pursued the mender of roads, on tiptoe and in a low voice, “withdraws; all the village whispers by the fountain; all the village sleeps; all the village dreams of that unhappy one, within the locks and bars of the prison on the crag, and never to come out of it except to perish. (ch 15, p. 109)
The revolution occurs in whispers. The revolutionaries are often whispering, and the revolution itself is a metaphorical whisper—until it turns into a roar. The whispers at the fountain are spoken of repeatedly, as both a metaphor for and foreshadowing of the revolution. Madame Defarge also sits listening to “the whispering trees” (ch 16, p. 113).
The whispers are also related to the second prominent metaphor: footsteps. Sydney Carton pines for Lucie.
And one other thing regarding him was whispered in the echoes, which has been whispered by all true echoes for ages and ages. (ch 21, p. 136)
Lucie can never be his. Carton knows that he cannot have her, but that she is in love with Darnay. This poetic thought foreshadows his later sacrifice for Lucie.