noisy clutch packs in rear is my guess. Get the fluid changed. How many miles? I changed mine at 1k and it was disgusting. You need the special part # fluid(in owners manual) with GM additive.
'15 Z/28 #325 black a/c & stereo. 2:10.9 VIR Full 1:44.74 Summit Point Shenandoah Circuit
'12 ZL1 10.52@131 1.55 60' 2:13 VIR Full (sold)