I've had the same problem. The way the vent is located in there is that the plastic peice coming from the outside of the vent that you see is a funnel that inserts into the duct that is behind the console.
They outline this funnel with foam that overtime corrodes and becomes brittle and eventually gets blown away whenever the heater/ac is on.
What I did to fix this was get foam double sided tape (3M Brand). Took out the vent (it's held on by about 4 screws behind the two plastic panels that house the switches and behind the radio and accessory controls) and cleaned off the old factory foam. Then took a strip of the double sided foam tape (about 1.25 feet length) unpeeled one side of the double sided tape and left the other side still peeled. I stuck the 1.25 ft single strip of double sided tape on the edge of the funnel that inserts into the ducts.
The foam of the double sided tape acts the same as the black foam porsche used. This prevents it from rattling and cured my problem.
Here is a link to the double sided tape.
http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1613135136
You can find them at any Ace Hardware or Home Depot for fairly cheap. I'd recommend 3m because they're heat resistant and the adhesive is better quality with 3M products.
If anybody gets confused or if this is common problem for boxster owners I can post up pictures as a DIY.