the silence was at first complete , but gradually as the roar of his own heart and lungs slackened , laurence began to be able to hear more clearly the small noises of the night , the rustling of mice and leaves ; the faint and far-off sound of the birds singing in the palace aviary beyond the inner walls ; the irregular rasp of tharkay 's knife against his whetstone : he was polishing the blade with slow occasional strokes , to draw no attention .