Rigid and quiet. Those are two words that manufacturers increasingly use to describe virtually any new or significantly redesigned model. Ford is using what it calls Quiet Steel in its SUVs and Buick has been promoting how its Lucerne beats its Japanese rivals in the area of sound insulation. Now you can add a midsize Saturn sedan to the list of low-decibel rides. Despite rolling on 18-inch Goodyear tires and its classification as a non-luxury vehicle, the 2007 Aura is surprisingly quiet, allowing for easy conversation at highway speeds and providing a welcome respite when navigating through snarled traffic after a hectic work day.