Well, I must say that my own loud "CLACK" at start-up has now gone away completely...at least since the last several times I have cold-started it after a couple days. What's changed? I replaced the exhaust vanos gears on both banks of the motor.
I was in the process of replacing my valve cover gaskets and discovered the plastic retention cap on the exhaust gears only were starting to work themselves crookedly on the gear. Concerned, I purchased a pair of used vanos exhaust gears that were in excellent shape. They were the same mileage as mine so I installed them, slightly timed the engine, closed everything back up, and I have not heard that sound ever since.
*major shout-out to forum member DeansBimmer for selling me the parts and helping over the phone while I did this in the garage.
Of course i'm not trying to stir up a scare and say those plastic/teflon covers will come off and damage the engine in everyone's cars, but they can certainly pop loose over time and make noise. Worth a look next time you change your valve cover gaskets.
...now, if I could just resolve the 2008 MT clutch rattle in neutral while parked
GM