bascially any thing that can knock i have removed or tightened...air box horn etc.. catch tank, matt did have a look found that everything was with in the right tolerances and tight bumpers are sucured still need to do rear bumper just not got room or time to make the brackets any ideas? ...Have you removed the resonator box from the inner wing?
If it's still in their it can knock (mine used too)
My first ZS had a rumble that started on at about 5k on the offside too !! The dealer said it was tyre rumble and I just lived with it until I sold the car on. My current ZS also has it but it's less loud than I remember my first ZS being. I'v put it down to design of the drive shaft, i'm sure the carrier bearing bolted to the block transfers engine block vibrations down drive shaft on the offside!!Had a rumble from o/s/f in mine from new. Worse in freezing conditions. Factory engineers even failled to find it with chassis ears fitted.