2006 Dodge Stratus underneath car

That is typical of a metal heat shield cooling down and contracting. This was common on a lot of cars in the '70s, but today a lot of factory exhaust systems use parts that are welded together instead of clamped. The recent work may have changed the position of something just enough to make the movement cause a ticking or snapping noise. Other than the minor irritation, that noise is harmless.