I'm trying to resolve this exact issue right now. Everything was fine until I put in the LCA relo brackets. About two weeks after installing them, I got the dreaded clunk. Sometimes it feels like it's coming from front of the drive shaft, other times it comes from the rear right beneath the back seat where the UCA is.
My adjustable UCA came loose once a few months ago, and allowed enough play to make almost an identical "clunk" over bumps etc.. however this time I've triple checked everything: UCA is tight, Panhard bar is tight, LCA's are tight etc..
Also, it appears to happen far more once the car is hot! (cold weather it's hardly noticeable). Could it be the stocker drive shaft?
Black 2013 GT Premium | Track Package | Recaros | GT500's | JBA H-Pipe | Borla Over-Axles | OEM GT500 Spoiler | Koni Yellows | Steeda CAI/Tune | Steeda Ultralite Springs | Steeda Adjustable UCA/PHB | Steeda Billet LCAs | MGW | Forgestar F14's