Vectra 97 heavy knocking noise from rear

Help,

I have a 97 Vectra that has a knocking noise coming from the rear end.

I have a pit at home and have checked for anything obvious i.e. exhaust or heatshields, and there are no problems there.

I have replaced the LH and RH upper outboard swinging arm bushes (one was badly worn), there doesn't appear to be any movement in the lower outboard bushes, or the inboard bushes.

The noise can be like a chatter on uneven roads, but is more like a thud when hitting a pot-hole.

Could it be the bushes in the rear dampers ?

Any advice would be much appreciated !.

Thanks,

Paul.