I have this rattle that is seemingly coming from my headset...

The bike is a conversion - all but the wheels + pedals was given to me, so it's all used, etc. I put new bearings in the headset when I built it up, but it was my first time doing it. I had someone helping me though, so I know I didn't fubar it..

Anyways, whenever I go over a bump, rough spot, etc, I have this rattle that sounds like it's coming from the headset. If I lift the bike by the stem/bars and shake it, I can hear it too. Sounds like somethings loose.

Any ideas as to what it could be, and how I could fix it?

thx.