My 2004 CS was plagued with the same loud wipers. Someone in this forum characterized it as "sounding like a herd of elephants". Finally, since my 3rd set of blades was installed by my Porsche service department at my recent 40000 mile servicing, I can drive quietly in the rain.
Do you clean the glass (with glass cleaner) occasionally? (Not just with screen wash). After a time road film builds up that is not removed by normal screenwash. The blades then don't move so smoothly and make noise, and can judder.