Hello - first time post here ...

I own 2006 CTS and had to purchase new battery this eve, because it wouldn't start after work. So, drove home and as far as i know, that issue of fixed. I live up a gravel road and have to drive slower, of course, so I noticed a loud rattle on drivers side, under the hood (I think). What could it be? The battery is on the passenger side and I had no other work done at garage when battery was replaced. I've never noticed this noise before this eve and don't have a clue what's wrong. The service tech did say he 'saved' all my settings for the car. I assumed he was talking about the radio and personalized info, although I have to reset the clock. Suggestions on the rattle? Did he make other changes to the car without telling me? I'm not a mechanic, but am willing to get under the hood to check it out. Thanks so much.