Got rear clonking noise on 53 STI from rear end passenger side. Happens first thing in morning reversing off drive then periodically normally on quick let off or bad gear changes. Just had rear shocks replaced for the second time under warranty by dealer as an attempt to cure (first time shocks failed completely).
Going to take back to dealer on sat morning but any other suggestions to what it can be so I can present a list. I've checked exhaust for clearance and I can wiggle quite freely with no contact. Drive shaft or any of those fancy links on roll bar?????????????????????????????//

 

Have them do a 'spanner check' over ALL the rear end bolts.
Could be the rear lower arm eccentric bolts used to set the rear toe-in. They would disturb these when doing the alignment.

It should all be nice and quiet down there.
Graham