Sounds like it could be the same issue i encounterd on that WRX.
The car would crash over every bump making very disturbing sounds.
After removing the wheels it was clear,
The right rear strut wasn't correctly adjusted and wasn't threaded down correctly pushing the shock assy up to the top mount giving about 1 cm of suspension travel, the springs where super compressed.
Some one randomly tried to adjust it with out any knowledge,
Front of the car head the problem other way round the springs where lose between the top mount and spring perch/bolt (it cut a trough 2 brake lines!)