Im sorry this isnt going to help you much but i just wanted to let you know so maybe some can help us both out, but im having kinda the same problem with my girls car.

She just bought a 00 Si and its lowered with weapon R sleeves and i cant tell what kind of shocks are on there but anyways when she goes over bumbs, more so over uneven surfaces, we can hear this noise in the back of the car, almost exactly like what you are describing. Hers might be more like a clunking kinda of noise, like it might do it 2 or three times. Its hard to explain. It doesnt rattle like rocks in a can or somthing like that, almost more like somthing is shifting or moving but instead of doing it smoothly it just kinda clunks to wear it needs to go.

Well now that i have made my Intelligent description of what hers sounds like, i hope that can help someone figure out what might be causing these noises.