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.
#917 A6. email@example.com 1.53 60' ATI/Lingenfelter balancer (10" ring), stock upper pulley, 90mm TB, ID850s,Rotofab/Lingenfelter intercooler reservoir, Cooler Logic HX, Lingenfelter idler relocation bracket, NGK TR7ix, 160 t-stat, Dewitts Radiator, Texas Speed 1 7/8" headers(no cats). M&H drag radials 325/45-17 & M&H 28x4.5" bias skinnies. Tuning by Frost at Performance Autosport in Richmond, VA (804) 358-2505