I had a similar "Clunking" sound when I'd hit a road imperfection while going slow. I recently replaced just about every joint, rod or bushing in the front end of my 126. After I replaced the guide rod joints, the noise dissappeared. Not really a hard job to do in terms of skill, but a good deal of grunt work. Check your joints out, they're positioned close to where your feet are.

Hope this helps.