Probably just crankcase vapors rising out of the engine. Since the truck isn't moving there is no draw created at the crankcase vent tube on the engine. Normally running down the road the air passing across the crankcase vent tube creates a draw on the tube and pulls the vapors out that way.......
Normally air is drawn in thru the valve cover and thru the engine and out the crankcase ventilator.

John