I recently built a hardtail, and right out of the gate I had one of those annoying "clunks" in the drivetrain. This one was random, and seemed to be coming from the BB or the pedals. As you probably know, on an aluminum bike a noise can be very hard to find, a clink can resonate from anywhere and seem to be coming from somewhere completely different. I changed out parts one at a time from another bike and could not find the problem. Today I finally found it accidentally, I use SRAM links and I had put an 8 speed link in a 9 speed chain. It is just slightly wider and would occasionally get caught in my derallieur. grrrrr that was a tough one!