"If you remove the hose between the airpump and the metal tube going to the air control valve (its supply line) there should be no soot residue inside it. If the engine is started, and any exhaust gas blow out of the is hose, the port air check valve is bad. This is what was wrong with my car. Apparently, the exhaust gas was blowing backwards through this check valve into the lower intake manifold near the turbos. It sounded just like a basic exhaust leak. What was strange when I listened to the leak noise, is that it would seem to disappear with some rpms. What was happening, is the airpump would overcome the exhaust, and the sound would stop. I think this check valve has been bad for awhile, and it finally "let go". "