Did you figure this out yet? Mine just started making a "fluttering" sound that fades in and out but only when driving down the road at speeds between 40-75 mph.
04.5 GMC Sierra 2500HD SLT LLY, 4X4 CCSB, Bill's Custom Intake Stage 4, S&B Powerstack filter,LBZ MP, 4" turbo downpipe into 5" exhaust, 7" MBRP tip, EFI Live w/ dsp5, Resonator plug, PVC rerouted and plugged, level kit, 33" BFG's, all-on mod
Head Gaskets, ARP Headstuds, Full EGR Delete, Aftermarket exhaust manifold, Race Valve, CTS INSIGHT
AIR DOG II Lift Pump, Smoked tail Lights, Smoked cab clearance lights, Lift kit, and Built trans.