Good to hear your under warranty. I would first try driving a similar car to see if it is something common in that model year. Just find a similar model at a dealership and see if its the same noise. Also, do you know the previous owner? I would make sure they didn't put any mods on, ie.. exhaust upgrade, flywheel, or anything else. Then don't hesitate to take it to the dealer.
Last thought and its a long shot. could it be tire noise? I have heard some strange noises coming from tires. try driving on different pavements. Usually if its tire noise it changes frequency the faster you drive. Kind of like when you put baseball cards in the spokes of your bicycle when you were a kid(might be an American thing
).