it could be several problems main 3 to start with are spark air fuel
most likely your fuel lines or filter could have build up or pressure loss
also a bad wire to starter or your air filter could need replaced so check air and battery and fuel
fuel pumps go bad after sitting for a while, check that
Leaking radiator? Leaking or loose radiator hose?
It depends what you are sitting on or in.
Change the fuel filter first.
Standing
A pair of small drums played with the hands, usually held between the legs while sitting. They are smaller than congas and djembes. A bongo is a animal, drum, brand of coffee & a truck.
check the main ground from your fuse lines
Think.
If its a motor knock it will do that while sitting/reving engine not just driving. If it's only while driving try different things to check it out. Does it do it over bumps?(parking lot speed bumps are a good test track) Maybe only when cornering? Try pushing up & down on the truck while its sitting and see if it does it. Does it shake also?
When sitting on the toilet. While stand with one leg up on a bed or bathtub.
In short diesel trucks provide better hauling capacity while gas powered trucks are better for acceleration and speed. There is some more information on the differences here: http://www.ehow.com/facts_5529606_diesel-vs-gas-truck-engines.html
In a chair.