In my case, I was chasing knock at about 1800-2400rpm when cruising at light throttle. Changed plugs, injectors wires and made modifications to my timing and fueling. I pulled the coils and noticed what looked like carbon tracking. Closer examination showed cracking of the coil casing allowing the voltage leak on a particular cylinder. Another coil was starting to show similar issue. The MSD coils were red so the tracking showed up more readily. With stock black, u may need to see if u get carbon on fingers when handling coils.