I'm working on a similar problem with my 99 Boxster. All the forums I've read have had different suggestions and many different solutions. Most point to the air/oil vaporizer that's part of the emissions control (I changed mine, wasn't it), serpentine belt (I sprayed mine with belt dressing, no change), I even read a forum where a guys spark plug was loose (haven't tried that yet) and the noise was from air blowing past the threads.
Other potential points are water pump, accy pulleys etc, etc...I suggest do the cheap ones first and knock-em down or take it in to a dealer and pay them to find it, then fix it yourself. Me I'm stubborn and am gonna keep fighting the fight. Best of luck, let us know what it is when you find it.
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