It does sound more like rod knock to me than valve noise. One trick people use to diagnose rod knock is to disconnect one spark plug wire at a time. If the noise suddenly goes away on one of the cylinder disconnect, you've at least narrowed it down to the cylinder that is having the issue.
X2
What year is your truck?
This is not a "cam timing" issue. It COULD be:
Valve clearances
Rod bearing
Oil Pump clearances
Power Steering Pump clearances/noise
Idler pulley on belts
Tests that can help isolate:
Pull the spark plug wires one at a time to isolate a cylinder to see if the noise stops (indicates a rod bearing or wrist pin issue)
Use a mechanics stethoscope to touch multiple places on the engine while running to more directly locate the noise.
Change the oil to see if it affects the noise level (how many miles does the oil have on it?)
Loosen the belts and check the idler / tensioning pulleys to eliminate possibilities. (retension when done.)
Check your spark plugs to make sure they are all gapped correctly and see the condition of them. Take a picture of each as #1, 2, 3.... and post so we can assist in diagnosis. Clean=head gasket (steam cleaned), light tan is good, black and "dry" is running rich fuel, black and wet is oil fouled, dark brown is OK with some oil burn.
How does your coolant look?
When it starts cold is there any smoke from the tailpipe? Color? How long does it last?
My first thought was PS pump, but the noise appears more pronounced as you move to the rear of the engine above the valve cover and its a low enough sound that makes me think rod bearing or wrist pin. Thinking #6, but usually (from what I read here) is that #1 is the typical failure for rod bearings when they are a problem (which is rare).
A broken valve spring would get MUCH worse as RPM increases and will cause more of a "miss" than a "knock". Typically, an engine won't swallow a broken valve spring, but it depends on the failure. Valve springs rarely fail unless you are running REALLY high RPM.
Good Luck!