S60 rear suspension noise

I have started to notice a problem with the rear suspension in my 2002 s60. If I go over bumps/pot holes with some speed, the back of the car give a "thump" noise. If I go over the road disturbance at a slow speed - no noises. I visually checked the rear suspension and did not see anything amiss, so my initial thought is that the rear shocks are shot. But I thought I would ask the forum to see if anyone has any different experiences or ideas.

thanks