There have been quite a few cases of creaking dampers - these have been replaced under warranty. There is a PUMA case out there for it, but I can't find the number for it right now... if I remember I'll dig it out tomorrow.
Other causes for the squeaking noise when bouncing the rear of the car - suspension bushes, although very unlikely on our cars as the bushes are more like elongated rubber bump stops that act as secondary damping (much more flexible than an ordinary bush). Could be the rear antiroll bar bush that's dried out - needs more lubrication (and wait for the inevitable snide comments here!!).
Final cause would be that all the welds on the back of your car have failed and it's about to fall apart...
Take your pick!!!
