At least literally, what the speaker is dreaming about is apples and picking apples. In his dream, he keeps going over and over the things he has done during the day. He sees the stems of the apples, he feels the ladder under his foot, he hears the apples being dumped -- he's basically reliving his day.
Figuratively, you can say that the speaker is thinking back over his life, reliving his whole life as he is getting older and nearer to death.
I would say that the word "rumbling" in this case is an onomatopoeia. The word "rumbling" itself sort of sounds like the thing that it means.
The poet has been picking so many apples in his lifetime that he still feels every movement and scent of the events. The pressure of the ladder rungs as he had climbed them, he can still sense. He remembers how he had prayed for a big harvest, just as all farmers would, and he had gotten the great harvest, but it came at the price of extremely hard work.
The man is now so overtired and relieved that it is over and picked. The season has ended. However, all the senstations that have been grinding at his body and tantilating his senses are still present. It will take awhile for the reality of the feelings to go away. He is still having trouble thinking back on the apples that had gone to waste on the ground as he tries to rest.
I agree that the rumbling is a sound and is onomonopia.