Mine makes the exact same noise, it only does it while starting in
reverse from a dead stop, the noise happens mostly when
backing into my garage. But sometimes it will happen on level
ground. The noise is a very audible metallic "clunk" and it is not a
noise that a strut or swaybar would make.
My speculation is that it has something to do with driveline lash.
With that being said my car has 23,000 vey enthusiasticly
driven miles on it, for 16,000 of those miles the car has been
modded. I have yet to experience any issues with shifting or
abnormal noises besided the "reverse clunk". I don't feel any
need for ME to worry about the noises that MY car makes. This is
my 3rd Impreza. All of them have made one strange noise or
another and not a one of them has given me any problems.