2001 Citroen C5 Rear noise

Hi. My 2001 Citroen C5 seems to have a metally clunking sound coming from the rear. It sounds like it's coming from the passenger side (UK) mainly. I have changed the drop links a the front ad i was told it was that, no luck there. I have added LDS fluid to the reservoir (it was low) as I was told it could be that, no luck there either. Could it be the trailing arm? Bushes? or any other fault as it's starting to drive me mad. The noise only happens if I go over SEVERE bumps in the roads, or speed humps, normal road bumps seem fine. Please help.