what can cause it:
1. shock mounts, you'll notice it more on bumps and potholes.
2. trailing arm bushings, while going 40km/h slam on the gas... if the bushing is gone you'll usually feel the back of the car jerk to one side.
3. subframe bushings, you'll notice the knocking whenever the rear subframe is put under stress. bumps, potholes, hard braking, hard acceleration. it's usually a very quiet THUD sound.
4. could be any one of your mounts, or differential bolts.
Also be sure to make sure your spare tire is secured, as well as your car jack. I remember I once had a small bottle that fell down there and was rolling around whenever I hit the brakes... sounded a lot like a dying RSM.
I've had a thumping sound at the back of my car since I ran over a 4x4 log at 80km/h... fell off a pickup truck right in front of me, and the car was airborn. Looks like it's a torn subframe bushing... I just have to keep tightening that subframe bolt every year or so, till I can afford to get it fixed properly.