And plug wire number 7 & 8 should be well seperated from each other to prevent induced firing. I'm sure your aware of this but I've had it happen. Sometimes the most obvious things are the culprit. From the sound you describe it does sound like spark or something arcing. Thesals had a good suggestion about running in the dark and looking for a spark. That and the fact when you remove the wire the noise stops. I've had new wires cook being to close to the headers and cause this. The arc sounded like a bic pen being tapped on your knee...Or the head gasket but I'm more swayed towards the spark thing. G.