You asked two questions. Unfortunately, enotes only allows you to ask one. Please do not ask multiple questions in future. I have edited your question accordingly.

From the very beginning of this rather frightening Gothic classic of Edgar Allen Poe, the narrator who tells us his tale places emphasis on the senses and how his "disease" which goes unspecified, had actually "sharpened" his senses. Thus it is that in the first paragraph he makes a rather bold claim that perhaps is the beginning of our suspicions that he may not be entirely reliable:

Above all was the sense of hearing acute. I heard all things in the heaven and inthe earth. I heard many things in hell.

Throughout the story this acute sense of hearing is returned to, especially as he hears the old man's heart, both before and after his death. This acute sense of hearing clearly comes back to plague him as in the last paragraph, whilst he is talking to the officers investigating the sound of the dead man, he hears once more the sound of his heart:

Yet the sound increased--and what could I do? It was a low, dull, quick sound--much such a sound as a watch makes when enveloped in cotton.

The ambiguity of this story is created by the fact that the officers never hear this sound. It is only the narrator with his acute hearing, leaving us to ponder whether this sound emerges from heaven--or from hell, as the narrator tells us he can hear sounds from both locations. Either way, it causes the narrator to confess his crime.

What is the sound the narrator hears in the seventh paragraph of "The Tell-Tale Heart"?

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.