I thought lifters early on as well, so I pulled them all out and tested them. They were filled with old nasty oil, but still held pressure fine, so I cleaned them up and put them back in. I also pulled the plugs and found that the #6 plug was broken and missing the ground electrode. I looked for it in the cyclinder as best as I could through the plug hole, but couldn't see it. Then, I replaced all the plugs and fired it back up and the sound continued, so I assumed the issue must be something lower down in the engine. More then likely it was a piece of the old broken plug that was banging around, and eventually made its way out. This weekend I plan to pull the exhaust off, and see if I can find whatever it was sitting at the entrance to the CAT.