It costs you nothing other than your time to call the dealership and discuss any issues with the truck & schedule an appointment. As @oldmann stated, a rattle would be covered under warranty if the dealership can duplicate the problem and if they know how to fix it.
Basically, if you feel there is something not right with the truck other than normal wear & tear, the warranty should cover it free of charge. Until you get to understand what is & isn't covered, ask the service advisor if you will be charged before confirming your appointment. It's also a benefit to be aware that not every issue can be fixed upon its discovery. It may take GM months or years to fully resolve certain issues & come up with a proper repair procedure. This day & age, the dealer is often at the mercy of GM when it comes to fixing certain issues (think of things more complicated than a rattle).