I had it as well. the theory I heard is that it's the chains slapping against the tensioner pads on start-up due to insufficient oil pressure. a check of the oil filter will show the little green bits of tensioner pad that result. Porsche have updated the pads with a harder material that are a good upgrade if you ever have to go in there (worn pads increase cam deviation which you can track with a durametric tool - 6 degrees is limit as stated by Porsche).
harder pads don't stop the slap however. I tried different weights of oil to try and maintain a film on the pads to no avail. there is a little known mod you can do which fixed the issue for me. Porsche updated the oil pressure piston on these cars; on earlier cars the end of the piston was squared off, the updated piston is beveled. the update requires a new piston, spring (you could probably reuse your existing, but a strong, new spring is probably better) and crush ring and is a 15 min fix for about $15; the beveled piston seems to bring the start-up oil pressure up quicker and fixed it for me.
it's part #5, 25 and 26 (follow the link for part #26 and you'll see it's updated to a 997 part) on the diagram below:
Oil Pump
the plug is on the bottom of the oil pump and is easily accessed from under the car at the front of the engine (you have to remove the underbody panel to get at it). there is a diy on the interweb if you search for it.
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