I got the same thing. It was the nut (maybe 14mm) that sits just below the adjusment screw. It came loose. There should not be a gap between the nut and the top of the pillow ball. I had to use to jacks to it re-tightened, one to lift the car the other I used to lift the suspension arm just an inch or two then tightened the nut. If you look on page 9 of this pdf it's the nut just below the adjustment **** cover. This might not be what your noise is, but if it is just trying to help.
http://www.tein.com/ti/inst/ks28u.pdf

A little noise is normal. Clunking is NOT! Regardless of what people say, Clunking can be fixed. I have ridden in several cars with Flexes that do not clunk. Every piece needs to be tourqued to spec and frequently needs to be retourqued after a few thousand miles. Clunking might be normal iwth Ractive coilover sleeves, buy not with Flex. BTW I have had both APEX and Tein RAs on my own car and have ridden in cars with various other sets. The Tein Flexes are good quality and if they are clunking, they are loose or broken. It doesn't take much to make a clunk.