Engine noise

Please listen to the sound in the attached video clip.
There is a noise that I did not notice before I changed the spark plugs, spark plug wires, valve cover gaskets, alternator, A/C compressor, serpentine belt, and idler pulley. It seems loudest at idle speed. It sounds almost metallic to me, but it reduces to barely audible when I disconnect the spark plug wire to the center back cylinder. I thought it might be a spark arcing to the engine, but it continues quietly from near the firewall when the spark plug wire is disconnected. My only other guess is air/fire escaping through the threads of the spark plug. Suspecting a cross-threaded spark plug, I removed the back three new spark plugs and reinstalled them with anti-seize grease. They all came out and went back in without any resistance until tightened normally. I even used a torque wrench to make sure I did it right the second time. The noise seems not quite as loud now, but is still worrisome.