Some possibilities are squirrels, rats, racoons, bats, carpenter ants, wasps or hornets but it could as easily be 'none of the above'. Additional investigation and research will be required to narrow it down further.
Not necessarily. The cracking noise that is found in houses and apartments, or wherever someone may live, is generally the structure settling. A very common thing for newer homes of any type as all things tend to move when they have been there for short to long periods of time. Not likely it is bugs.