Hi Guys, I had this same problem in my 05 WRX. Took it into Subaru (under warranty) and they diagnosed it as the strut (rear right).
That fixed the thumping, but it's been barely 6 months, and now it's back! I feel and hear it well under 20, mostly less than 10 specially when I back up.
a) obviously, it wasn't the strut. or, it was the strut and something else.
b) should I make a fuss, because they haven't really spent enough time on the diagnosis, have they...
Any advice before I take it in again is much appriciated.