Knocking Noise from Rear Wheels

Hi

I have developed a knocking noice coming from my rear wheels. It is OK (i.e. no noise) on a smooth road but on a bumpy road its quite bad.

A few months ago I had this on the front wheels and they replaced my steering arm links.

Any ideas as to what I need to change on the rear? Could it be the balljoints maybe?

TIA Oli