Whenever I hit even a small road bump, I'll hear a loud, metallic CLANK that appears to be coming from the rear end. In fact, it make almost the same noise as if I suddenly engage the freehub, which makes me wonder if it might be related to the freehub. It only happens when I'm on the bike riding, if I just raise the rear end off the ground and and drop it I can't hear it. <-- Wrong, still makes noise if I'm not on the bike.

I just rebuilt the rear hub (older shimano) and it is well adjusted, no play. No play in headset, front hub or BB either. Any ideas?