2007 Pontiac G6 Under the car noise

That's like asking why I'm in pain. I might have a hang nail, a stomach ache, I cut my foot off with a chain saw, or my ex-girlfriend came back. There are hundreds of parts that can wear until they rattle, thump, or vibrate. You have to provide some clues as to the area of the car you hear the noise, under what driving conditions such as speed, direction of turning, braking, bumps in the road, and things like that. With nothing else to go on, your best bet is to have the steering and suspension systems inspected at a tire and alignment shop. The people there are experts at finding the causes of noises, vibrations, and bad tire wear. They'll start with a test-drive to get all the clues they can find.