Described, rather accurately by Gaz, as the noise you hear from the bed springs mid-session. This is what my back end sounds like. I replaced the bushes in the rear trailing arms (3,4 and 5 on the drawing) about six months ago for Powerflex. Could the squeaking be coming from the upper bushes being worn?
Spring pan can be powerflexed. You can also replace the outer bush with a Mercedes spherical bush ie an original rather than the powerflex, keeping the original geometry and movements of the springpan arm. the bush is available from Eurocarparts for about £15. a quality Lemforder part. I have been unable to track down a replacement bush for the inner end of the springpan arm although it must exist. However I dont think this is a spherical (or rose joint as some incorrectly call it), just a normal metaelastic bush.
Maybe your Powerflex is squeaky and your rubber needs some lubrication?