I have the same issue. I even had Borla send me a new exhaust because I couldn't find out what it was. New exhaust still rattles only after long drive or after heavy duty fun
. Mine is also on drive side. I can replicate the sound with certain amount of throttle once it starts. It was gone for a long time then today I drove it 35 miles and toward the end it came back. I've gotten under the car and looked at everything one day and nothing seems close enough to touch even if it did flex under heat.
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2012 Grabber blue, Auto, Eibach pro street S coilovers, MM caster/camber plates, Borla ATAK cat back, off road X pipe, A.E.M CAI, SCT-x4 Mike Roush tune,SR LCA's, BMR Relocation brackets, J&M Pan hard bar, UPR Catch can, SVE 15x10's, Mickey T's 275-60-15.