I'd disable the ign & crank it & see if you an pin it down with the stethoscope or heater hose. You might spray some dish soap/water in there & see what bubbles up & I'd try it with no ign then if no results start it & see if you see anything & I'd do it cold so the heat doesn't vaporize the liquid & make diagnosing this more difficult. they say exhaust leaks and some types of valvetrain noises sound incredibly similar. Holler how it turns out.