From my experience. I am sure someone else has more info. But, I brought mine to the dealer for the same thing when I first got mine. The lead SVT tech that it is the rear end locker/axle and that it is normal... He said he could lube it a little more and it would maybe quiet it a tad. But, not get rid of it... He also said Ford sent a letter to all dealers explaining that this is normal.
2011 White GT500 SVTPP/Nav/Glass Roof
2.6 pulley, VMP Tune, C&R HE, 3x Stock look Intercooler tank, Eibach Pro-Street Coilovers, Shelby sway bars, BMR Adj Pan-hard bar, BMR Adj UCA, BMR Billet LCA, BMR LCA relocation brackets, Maximum Motorsports Camber/Caster Plates, FRPP Resonator Delete,NXT Hood Struts