I had something similar after I put Cusco caster arms and some thick whiteline swaybars on.
The stock endlinks are a little weak and my whole swaybar had shifted a fraction over to one side, and as the suspension went up and down the caster arm was hitting the swaybar. The swaybars are the thickest non-adjustable ones that Whiteline make for the GTR, but they fit on my GTST.
I bought whiteline locking rings, but they didn't fit... in the end, I loosened the swaybar U-brackets a little, and crowbarred the swaybar back to the middle, tightened it all back up and now there is no more knocking.
Let us know if this helps!