What do the people in town say is the cause of the “Monadnock roar”? Even though Amos knows about the natural cause, what does the roar mean to him?

Amos believed the "roar" was the voice of God.

He remembered hearing Parson Ainsworth and a group of men talking about the Monadnock roar, one day, and they had said that all mountains made such a sound at times, mountains with forests growing up their slopes and summits that lay in the path of storms. The wind blowing along the mountain and meeting another current blowing down the mountain could cause a roar loud enough to deafen a man save that its duration was brief. That was what he had heard them saying in the town. But to Amos on the mountain what he had just heard was the voice of God and because he had asked to hear it he knew that he must heed it.