WARNING: This page is about a different car, the 2002 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
Inspect the exhaust manifold mating surface for flatness.
Inspect the exhaust manifold for cracks or damage.
Replace the exhaust manifold if necessary.
Fig 1: Inspecting Exhaust Manifold Mating Surface For Flatness