Is this a rhythmic bump everywhere, even on smooth cement... or just a jarring rattle?
In the first case, it could be a tire/wheel issue, for example a bead not seated, wheel dented or out of true, skid mark, etc. I once tried a full sized Park Boot, and could feel it bump.
In the second case of a rattle, could be tire pressure, or even in the head. Perhaps thicker bar tape or gloves? Or a headset issue?