I remember looking this up either on these site or Rennlist. It did have something to do with a pump or vaulve on start up and it is normal. I look at it as something similar to us when you wake up in the morning and clear your throat.
abe

 

Mine does it too, but I'm quite certain it's either normal or the majority of the 3.8L cars suffer from this problem. I happened to be hanging around the Porsche dealership one morning as I was waiting for my oil change to be completed and, for whatever reason, they were moving a bunch of their cars around the lot. They were starting them cold and I could clearly hear the 3.8L S cars making the same/similar noise. Most if not all the cars were brand new, demos or lightly used 997s and all sounded the same/similar. I stopped worrying about mine after that day...

 

Same thing with my 2008 2S. 20K miles.

 

If you are referring to the popping noise that goes away in a few minutes, it is due to an emissions cycle. The ignition timing is retarded and the air pump is acitvated to heat up the converters to minimize the increased exhaust emissions from a cold startup. If it is a buzzing or high pitched rattle, it is likely a heat shield or exhaust system component.

 

Same here 2006 C2S. I just use that 2 min or so to warm up my car.

 

When I start my C2S up, I don't hear any popping or rattling in the exhaust. The exhaust sounds as connected to the frame as ever, but the note does tend to "burble" for about a minute (sorry for the techical term).

 

My '07 C2 does it too. It goes away after about 20-30 seconds.