the scientific reason why empty cans make the loudest noise because empty cans has more space inside, allowing sound to echo loudly. applying that to people, the meaning of the idiom is that people with empty brains (stupid or have no knowledge) makes the loudest noise (by chatting with their seatmate or just being overall noisy).
“to autumn” is rich in imagery, evoking the perceptions of sight, hearing, smell, taste, and touch. each stanza highlights one of the senses. the first stanza especially evokes the senses of smell and touch. the sharp smell of the early-morning mist, the mellowness of ripe apples, and the sweet-smelling flowers attracting bees all work together to tempt the reader into believing that summer will never end. nothing appears static in this stanza; the fruit, the nuts, and the honeycombs swell, bursting into ripeness, spilling out of their shells. keats emphasizes the sense of sight in the second stanza by inviting the reader to see autumn as harvester, her hair “soft-lifted by the winnowing wind,” checking, cutting, and gleaning the crops. the sights evoke a certain lassitude. autumn moves slowly amid her stores; she sleeps, “drows’d by the fume of poppies”; idly, she watches the “last oozings hours by hours.
because this poem is exquisite, readers are often oblivious to the many images that keats employs. indeed, the poem is loved by many persons who might wonder if knowledge of the images could add anything to their appreciation.