What warranty do you have? Did you buy it used or new and how many miles are on it.
If you are still under the 36k mile warranty then pretty much anything under the hood will be covered.
As far as what is rattling, there just entirely to many parts that can loosen up under the hood that can make a noise. My advice, don't wait, get it into a garage and make sure everything is tight so nothing will fall off and either break something worse or cause a something to stop working that you may need.