Today i received a recall notice for my 2000 3.8 leSabre. Under low battery conditions a faulty and leaking fuel pressure regulator can cause a backfire into the plastic inlet manifold. Evidently GM will inspect the fuel pressure regulator and fuel rail and replace if neccesary.
My guess is the fuel regulator is made of the same plastic as the "already replaced" inlet manifold.
The noice also says that if the engine cranks slowly and you hear a big bang stop right there.