Opinions on the noise my 2006 Explorer 4.6 v8 3V is making

That knocking noise is only when in gear with a foot on the brake pedal, basically when the engine is under load. I can't hear it once I move, and the engine is smooth. It does not come up when cold, only after prolonged driving.
Maybe it is cat, but not the heat shields. It also idles very unevenly at the stop light.