Have you used Lithium grease on the topmounts and springs? as this is trait with coilovers..
I have the same systmoms then took them off and greased them up all is fine..
Edit: ignore that...It will be the exhaust going over the rear beam..When its that low...Like mine in the pic it knocks...How i got over this jubliee clip and some Heatwrap...hasnt knocked since..
This is how low mine is and it used to knock...Just check the clearest with the exhaust also is it a milltek?