A sound as of quick, low breathing.

In the open such a sound would not have been audible, but in the ghostly darkness of that strange prison he could hear it clearly when he listened.

Sometimes he could distinguish the momentary pauses between the breaths and sometimes the faint sound seemed continuous.

As he listened in silent, awful terror, the thumping of his heart seemed to interrupt the steady, low sound.
It was not normal breathing surely, but it was the sound of breathing.
He was certain of that.

He thought it was over near the car..