Knocking noise from rear of Boxster

[dctvr]

I have a late 97 Boxster and am experiencing a light knocking noise when on very bumpy roads (particularly the red slip-resistant finish being applied everywhere these days). The knocking is not apparent when I am on the throttle. I have tightened up the cabriolet top tensioning ties as I have heard that this can cause problems and am going to remove the cover over the gearbox in the rear luggage compartment this weekend, as this makes an enoying rattle. I have had the car looked at by a Porche centre who say that they cannot fins anything wrong and that the anti-rollbar bushes etc. are fine. Does anyone have any ideas? does it sound transmission related?

[BliarOut]

I take it you have completely emptied BOTH boots