i second checking your front hub if it's not your headset. when i first built my iro up, a noise developed. i went freaking nuts trying to adjust my headset. after much headset headachce, i realized i had a locknut on my front surly hub break a few threads out, which made it appear to be tightened enough rotationally, but the locknut could rock a little bit laterally due to the threads. i replaced the locknut and the noise went away.