Well, I don't want you to hurt youself or your car, but when I'm looking for noises like that I usually have to get a little physical with the car. Since it only makes a noise on speed bumps etc, you'll probably need to be that hard on it. I usually push down with one hand and pull up on the fender well (or corner of bumper) with the other. To get an idea how hard; sometimes the car will bottom out on the down stroke and catch some air on the up stroke. An easier, but not usually very effective method would be to jack the car up and down slowly, listen for noise and watch for movement. Make sure the battery and radiator are secure, and it wouldn't hurt to check the motor mounts as well.