Hey Andy,

Id say first thing is how familiar is the bikeshop with single/fixed gears since that is a rather unique hub set up. Iv ran into a similar problem with my fixed before. One thing id suggest is check you gear ratio. High ratios and tentions create alot of torque. On my bike i found that the high torque created a similar noise to come from my chain and spokes and actually contributed to a broken spoke. Is your ratio higher then 3? Also when does it happen when your riding, all the time whether your on the flat, decending, and climbing? If it only happens when your climbing or trying to accel on the flat id say it could most likely be a ratio related tention problem.