The one time I had that type of noise, it was because the headset bearings were completely trashed, as in the ring that held them was broken in several places, and the balls were just rollin' around in there. It also explained the shuddering on descents. Replaced both rings and it went away. Very annoying to diagnose, and the shop couldn't even hear the noise when I brought it in. I finally just had them take the front end of the bike apart, where they discovered the trashed bearings.