it could be just them knocking, common fault . I got grease nipples put in mine and that solved the problem. EricRX put a post on how to regrease sti shocks there lately too
Shocks knocking is a problem in all newage STi's. Subaru say new shock needed but its really just the greese goes off in the shock and there is no where near enough in there in the first place.
greasing the struts works for about 6 months
KYB rear struts can be had for £110 a side who make them for subaru in the 1st place, i fitted a set 2 years ago without any issues.
wee link below should help you out.......
i get the droplinks and bushes done next week 1st..also thinkin of just getting to just regrease them while he has the car.. well do tip 1 just spray it on and see how it goes.