Could be the exhaust. It's going to be hard to diagnose without a hands-on inspection as there are many possibilities. Use this link to find a good shop near you.
https://repairpal.com/repair-shops
Alex
RepairPal Certified shops can diagnose and repair your 2005 Chevrolet Malibu. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.