Take the covers off the AC vents and look in there with a flashlight and a small telescoping swivel mirror. Sounds like the AC run might not be sealed up. Make sure it's taped/sealed to each outlet, and make sure the ends of the run are sealed on both ends. If that doesn't help or take care of it, you might have to check the AC it's self to see if they installed it correctly. Some times the air box is not sealed.
As far as the floor squeak, I don't know what to tell you that would be a simple fix. A lot of them do it, including mine. I have learned to ignore it.