5. Lines 170-178: Why is the description in lines 172–174 in italic type? What does the narrator think the sound is? What is the likely source of the sound? I

The descriptions in lines 172-174 are in italic type to emphasize that sentence. The narrator describes the sound as a low, dull, quick sound, and a sound as a watch makes when enveloped in cotton. The source of the sound is probably his imagination or his own heart. Because he is nervous, he maybe started hearing things that aren't there.  

Explanation:

the aesthetic impact is that moment when you are relating spatially to the target. the "ai" occurs when you have an impression that includes both you and the target and is usually a spatial relationship.