Rattles on bikes can be really deceiving. Just because it sounds like it's coming from your headset doesn't mean it actually is. It probably is your headset, but it could also be anything from your chainring bolts, to your bb, to a flake or rust/paint/solder/etc rattling around on the inside of your bike. Take the bike to your LBS, get the headset adjusted properly - if there's still a rattle, you can at least rule the headset out.

I just watched three episodes of House in a row. I'm feeling all cranky and diagnostician-y.