I've just cured a similar problem which was very elusive and I only figured it out after totally sripping the rear interior. Although I could not find this on the workbench, it was a rear shock-absorber problem. I swapped the shock-abobsorbers left to right and the knock swapped sides. I've now replaced both shock-absorbers and the knock has gone.
Of course you may have a different problem, but try removing the entire rear interior, seat belts and all and see if the knock is still there. Another relevant point is that without any interior trim and fittings, it's much easier to pinpoint the locaton of a suspension knock.