Raise your wiper blades up, grab your passenger wiper arm near the splined shaft it mounts to and and give it a wiggle while watching the splined shaft to see if they move together, or independently. If they move independently, pop off the plastic cover and tighten the nut behind it that holds the arm to the splined shaft. Torque spec is 12.5 ft-lbs.
I found my passenger side wiper nut was completely loose on my 2001 boxster with 110k miles. Drivers side wasn't loose but was less than the 12.5 ft-lb spec so I tightened it and believe it's not going to loosen up any time soon.
Porsche Workshop Manuals > Boxster (986) F6-2.7L (2000) > Wiper and Washer Systems > Wiper Arm > Component Information > Technical Service Bulletins > All Technical Service Bulletins for Wiper Arm: > 3/00 > Jul > 00 > Wiper Arm Nut - Rattling/Rubbing
Bosch aftermarket wiper blade/frame set made specifically for boxster with the curved passenger side frame is working great for me. I recently also put on some aquapel on the windshield... Good stuff and the water practically flies off the glass!