what he said about the o2 sensor bolt. that ticking will keep going until the bolt pops out, the sound is it loosening itself. i heard that same noise from the ebay header i put on my eg8 a year ago, it had the top o2 bung in the engine bay, and one on the bottom of the header near the collector. i didnt tighten the bottom one all the way (forgot to) and going down the road i hear ticking for about 5 miles then POP!! bolt shot up into the floorboard scared the shit outta me. loosened itself out.