strange knock/clunk
 

I hope someone can help me with this. I've been having a knocking sound coming from somewhere on my drivetrain while I run in my 3 smallest cogs (7 speed freewheel) on my KHS '04 Sport. It repeats itself throughout a single rotation of my wheels/crank. It only happens when I haven't been on those cogs for a few minutes and stops after about 30 seconds. I find no obstructions behind the freewheel or on the threads. It doesn't matter the pressure I put on the pedals, it still makes the noise--so it's not likely a BB issue. It happens on every chainring too. It's not likely to be spokes as I've substituted different wheels and the same thing still happens--which means it's probably not the rear hub bearings either. The chain was replaced with a better one the day I bought the bike new (2 months ago) and I haven't found any tight links. It's only been in the last two weeks and I ride it every day. I'd take it to my shop (free service for life) but he just closed shop. Another KHS dealer in town is honoring the free service thing, but I can't stand the thought of being without my bike for a couple of days. Any ideas? If not, I'll cave in and take it in ;).

Thanks