I know a number of people with the Tein Flex coilovers installed. And most everyone I have talked to has the common "rattle" in either one or both of the rear struts.
While doing some front brake work the other day, Godzilla (Kyle) and I decided to drop the rear suspension and clean out the pillow ball. We used some WD40 and rags to wipe off the entire assembly. Reinstalled the rear suspension and guess what- absolutely no more noise.
Tein owner with noise, try this out if you get a chance and let me know if this helps.