Mine sounds like that too, but not quite as pronounced. Let us know what you find out. What is your mileage and what do you do with the truck. I have a buddy with a Chebby gasser and he has some bad tings and knocks, they are caused by carbon build up. Chebby dealer say its normal for the model and the only way to stop it is to clean out the exhaust valves and manifold, until it comes back. Possiblity?