Probably a worn out bushing in the suspension. There are a fair amount to choose between.
velocity
It actually can be from the rear axle being too low to the ground. The back of the car hits the speed bump. That was the case with my bro's car anyway
check shocks
Could be any number of things. I need more information
Maybe the stabilizer bar.
Can be a loose shock, or loose suspension parts. Can also be loose exhaust system parts. I had a car that made a front-end knocking sound on bumps. Any little bump would do it. Turned out to be bad struts.
I only know from experience that it is the rear sway bar, it took me forever to figure it out but that is what is wrong. The rubber O ring foes bead and it is then metal on metal that you hear when you go over the bump!!
ball joints
A ball joint going bad.
most likely a short in the secondary wiring system.
Check your sway bar links. I just had to have these replaced on my 2004 Tracker.
you need to check that the shock absorbers are not worn out or broken