Thanks so far for the responses!
Don, I'll double check the flywheel but, the sound is up front.
Bartamos, I believe last time I used the ol' gigantic screwdriver but I have an electronic ear at work that I'll borrow. I'm leaning towards the valvetrain as the knock is one half of what the crank turns. That would include one exhaust port leaking. I'll have a closer look at those stock headers. The only change they went through, beside coming off and on the motor, is me welding up the air injector bungs. It might be possible that I missed a spot.
