2000 Grand Prix

My wife has a 2000 Grand Prix which we purchased new in 2000. We use this as our family car. The noise level has increased in the car from the rear. I have checked the tires and exhaust and all appear in good order. Any ideas on what can be causing this to happen?

Could be the resonator. Its the long tube like muffler that blocks some sounds of the exhaust. Some people remove it to make their exhaust tone louder. So maybe its falling apart inside or maybe the mufflers have holes in them.