I have a 2006 Chev 1500 w/ the 5.3L V8. I've only had the truck about a month and I love it. However, while driving tonight I began to notice a new sound. When I'm accelerating I hear a noise coming from the engine compartment, or possibly under the truck, that sounds very similar to the sound a diesel engine makes. The sound goes away if I go full open throttle and if I let off the gas. It only appears when I'm doing normal acceleration. It was happening after driving for a while so it would not be cold engine knock. The only real mod the engine has is a cold air intake. I run 85 octane gas and have not had any problem before tonight. Does anyone have any idea what it could be? Maybe engine knock, heat shield loose, headers, exhaust, something else???