Among the thousands of miles of gold in soybean rattle, "rattle" refers to the way soybeans sway in the autumn wind, and "thousands of miles of gold" vividly describes the golden scene of mature soybeans and other crops scattered on the ground.
The description of the soybean bell is that when the soybean is ripe, the beans in the pod will become full and easily fall out of the pod. When these beans and other crops are scattered on the ground, they will form a golden scene and shine like gold. The scene is beautiful, giving people a sense of harvest and richness.