This chapter, amongst other things, narrates the first of Gatbsy's parties that Daisy actually goes to. However, although this party had the same lavish opulence about it that all his other parties are associated with, Nick feels that something is different:
....but I felt an unpleasantness in the air, a pervading harshness that hadn't been there before.
This harshness comes to light by the increasing antagonism between Gatsby and Tom and Tom's suspicions. This causes Tom to want to find out more about his rival and to investigate his background. Note his determination in what he says:
"I'd like to know who he is and what he does," insisted Tom. "And I think I'll make a point of finding out."
This fulfils if you like the feeling of unease that Nick experienced at the beginning of the party. The "pervading harshness" relates to Tom's distrust of Gatsby and his increasing opposition to him. Precisely at the time when Gatsby appears to be moving towards achieving his dream, Nick is aware of other forces that threaten to destroy it forever.