A whisper of sound, so faint it’s right on the edge of hearing. If the Doctor’s ears weren’t so sharply attuned to the breathless silence filling this place, he would have missed it and then… Well, it wouldn’t be a good thing.
No time to think about that right now though; their hiding place has been discovered, the only thing to do is run as fast as they possibly can, like the hounds of Hell are nipping at their heels.
Perhaps they are, in the form of the creature hunting them. The Doctor doesn’t intend to wait and find out.