theres an assortment of bushings you can fit to stop the thumping
start with the moustache bar donut ring bushings (superpro), upgraded sti group-n gearbox mounts, gearbox subframe support mounts (superpro), rear diff bushings (superpro), sti/cusco spherical lateral and trailing arms, sti tailshaft propshaft bearing mount and lastly the whiteline rear subframe lockup kit.

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Long live the WRC