Does the noise increase with engine speed? If so, without hearing the noise myself, my best guess would be a sticking lifter. Although a lifter makes more of a tick sound than a knock. Lots of time lifter noise will sort of come and go. My experience with it, the lifter would stick in an old Chevy 350 I owned, as the engine warmed up the lifter would free itself and the sound would go away. Sometimes the sound would come back on a cold start, other times I wouldn’t hear it. Generally, once a bottom end knock starts, I don’t think the sound will ever go away.
How many miles on the truck, and how long between oil changes?