Could be the top mounts, though when mine went I could make it 'knock' by "bouncing" the car.
Rear suspension bush may be the culprit. Had one go on mine last year. It went whilst towing my van. Knocked only over massive holes/bumps. Once unhitched it was knocking like mad.
Just forward of the rear wheel underneath. PITA to remove had mine done at an Indie as I was on Hols . £240 fitted I think it was, that was TRW part.
One of these :
