started my C4S us normal today, and immediately noticed a very squeaky timing belt of some form. couldn't locate it but it stopped once the car had warmed up. Never heard this before - belt needs changing or something more sinister en-route?
Noise will be more than likely from the drive belt which is situated in front of/below the airbox. Pretty straight forward to replace and not expensive if that is where the noise is from,check pulleys for wear/noise once belt removed.
Agree most likely a drive belt that needs adjustment or replacing. I just had mine replaced on a 996 C4S during a major service. You have a timing chain rather than a belt and if this were slipping (highly unlikely) it would most likely be terminal very quickly !