What do Scout and Jem hear that makes them realize that their companion isn't Cecil Jacobs?

From the text:

Our company shuffled and dragged his feet, as if wearing heavy shoes. Whoever it was wore thick cotton pants; what I thought were trees rustling was the soft swish of cotton on cotton, wheek, wheek, with every step. I felt the sand go cold under my feet and I knew we were near the big oak. Jem pressed my head. We stopped and listened.

Shuffle-foot had not stopped with us this time. His trousers swished softly and steadily. Then they stopped. He was running, running toward us with no child’s steps.