Have a look at all the bushes, and the spring rubber insulators. Look around, also it could mean that one of your shockers have died... One of mine did
My rear suspension does the same thing; it's the trailing arm bushes in my case (old and worn out), but it could be just about anything in your case. The best way to find out is to get under the car and have a friend bounce the back of the car while you listen for noises.