According to the narrator,
... I heard a slight groan, and I knew it was the groan of mortal terror. It was not a groan of pain or of grief—oh, no!—it was the low stifled sound that arises from the bottom of the soul when overcharged with awe.
I believe it was just this: the old man's groan upon realizing that there was something--or someone else--in the room, something that would eventually lead to his death. For an hour both of the men remained motionless and virtually silent: the old man waiting for something to appear, and the narrator--the killer--awaiting the perfect time to make his move. Unlike the other sounds the narrator imagines, whether it is the beating of the old man's heart or his own; or whether it is something else, like the "death watches" ("a type of small beetle that lives in wood and make a ticking sound"--like a watch); this time it is not imagined but the real breathing and the terrified sounds that come from the old man as he awaits the terrible consequences that are soon to come.