rattling sound

If you hear a rattling sound under your truck or in the area of the engine compartment when you press the accelerator pedal I suspect the exhaust system may have a problem, either a heat shield or muffler that has deteriorated, or a catalytic converter that is breaking up internally bu this should set a P0420 trouble code. I hope your rattling sound is not internally in the engine, check the engine oil level and get a mechanic to give there opinion. Get it looked at as soon as possible to save possibly doing more serious damage.

sounds like the catalytic converter....I had the same prob...more noticeable when first starting and would go away once warm.