It had followed them.

And it must have run also, and perhaps faster than they, for it seemed to be nearer to them.
At the same time, they began to perceive a certain noise of which they could not guess the nature.

They simply noticed that the sound seemed to move and to approach with the fiery face.

It was a noise as though thousands of nails had been scraped against a blackboard, the perfectly unendurable noise that is sometimes made by a little stone inside the chalk that grates on the blackboard.
They continued to retreat, but the fiery face came on, came on, gaining on them.

They could see its features clearly now.