2005 Prius questions.

These are issues im having.

My ac works perfectly. But it makes this loud grinding noise. Is that normal?

My mpgs are avg 38mpg. I get like 380 to a tank. Should I be getting more?

The car idles sometimes when I stop. But not all the time. Could something be wrong?

The car makes a weird noise when I shut it off... now what is that!?

The car drives great.. got nealy 100k on it. Its my only car I just want to make sure its gonna keep running strong.

Oh is there a place where I can look up maint codes?