Hear a "thump-thump-thump" coming from underneath your Cayenne? The drive shaft center support bearing could be bad. The rubber membrane that suspends the bearing in the bracket deteriorates and allows the driveshaft to wobble, causing the noise.
This part is not available from Porsche, but with some mechanical expertise and a little patience you can save hundreds over the cost of a rebuilt driveshaft.
Comes with detailed instructions.