Knocking Noise
 

I have a knocking noise from the rear left of my Boxster and I was thinking of replacing all the bushings with new Powerflex ones. Do you think this will solve the problem?

The knocking noise is present on uneven surfaces, and almost goes away then the accelerating pedal is pushed down.

Motor mounts, too, need to be considered.

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......