my truck makes that same noise. im trying to figure it out. almost sounds like its the CAT. like its all busted up and rattling inside. only thing is, if its the cat, why does it only make it at that specific load. if it downshifts, it goes away. if i let off the gas, it goes away. its driving me crazy and im hoping its not something serious motor wise. i have heard about the cam phasers on these trucks having problems and i know about the valve problem but the noise i am hearing is definitely not the valves.