Vitals: 06 C6 Z51 MN6 8000 mi. Stock other than axle back Flowmasters. I hear what sounds like spark knock under half to three quarter throttle from about 2000-2500rpm. Goes away after about 3000rpm (or I just can't hear it). Always premium gas and have tried switching brands with no luck. Has anyone experienced this? Suggestions before I take it to the stealership and let some schlep try and replicate the problem!?