[Porsche man]

Hi all. I have a 2000 Boxster S with 80k on the clock. The car has developed a noise in the rear that occurs on smooth surfaces, eg concrete and at reasonably low speed, and when I run over a small bumps, say a cat's eye (reflective marker). The noise can be described as a kind of dull rattle. Suspect it is a worn bush on the rear suspension. I don't want to replace all the suspension components at the one time when it may not be necessary. Question is where do I start, and what suspension component from the experience of others is likely to wear out first ?