Your headset's loose. It's easy to fix. You have to loosen both the bolts that clamp your stem to the steerer, and then tighten the bolt on the headset's top cap. Not too much though, just get it so that it doesn't rattle anymore, and then maybe go a tiny bit more. Then tighten the two steerer clamp bolts on the stem again, making sure the bars are straight of course. Those two bolts provide the strength that holds the whole thing together; the top cap just gets the stem into the right place.
Park Tools website will have a slightly more detailed writeup with pics if you need it.
Last edited by mander; 11-02-09 at 03:18 PM.