I recently developed an engine knock at start up. It's not a a ticking or pinging noise, sounds more like if you took a rock and tapped it on another rock. I would describe it as a moderate volume knock as it is apparrent with the hood up but cannot be heard from inside the truck. I've taken a stethiscope and is loudest when placed near the number 5 & 6 spark plug on the valve cover but can be heard from anywhere on the valve cover. It can bearly be heard when placed anywhere on the engine head. Once warmed up the knocking is silent. Would this be the valves being too tight and needing adjustment or something else?