My 2011 Cruze starts fine, drives, bit makes a almost nonstop rattling noise when it's running. Its power isn't like it used to be, difficult to get it up to speed on the highway. I think the mpg is down a little, but nothing crazy at all. Heat ND ac are working perfect, and it's not leaking a thing. I just don't know how to diagnose this issue. Where do I start?