[Steve Tinker]

To add to Flavor's comment, the knocking could be worn drop links, sway bar bushes, upper shock mounts & bearings, rear control arm bushes or coffin arm bushes as well as the shocks themselves.
To replace just the bushes in your existing suspension arms is very difficult without a suitable press. Its a lot easier to replace the individual suspension arms themselves....
Check out Design911UK view the online version for the specials being run at the moment......