[brighton911]

Gordon, it would seem your engine has been talking to mine. The reason mine is apart this winter is due to a very similar noise as you have described. For mine, on cold start up, quiet for the first 20 seconds then that slight knock you describe. It was present if I revved it and once warmed up, zero noise.
Some history, a case split rebuild 7k miles ago. 964 cams from John D, Craig Garret did the heads and rockers including bushings. New rod bearings with mains being perfect so left alone. Mahle cylinders light hone and new rings. Timing chains replaced as were all ramps. Runs like a champ with negligible oil consumption. Figuring I had a slightly collapsed piston, I jumped on a set of 98 P&C that came up for sale and they are going in this winter. But I have yet to find anything that would account for the noise. I have replaced the small end rod bushings as they were on the high end of OK. Tensioners are hard. 95mm P&C still check out as good clearance. I would really like to find the cause as it would be a downer to hear the same noise after all this work. Hopefully someone can provide a clue for both of us.

Interesting point jlex made about an exhaust leak. My noise is a very definite metallic sound so I don't think (but I wish) it was that. No evidence of exhaust leaks around the gaskets either and the system was tight at the heads.

Dave