You don't provide a whole lot of amplifying information (history, type of oil, how long you have had the truck), but very often that clicking sound comes from valves or valve lifters. If, for instance, the engine has not been cared for well and you put in a synthetic, it could have cleaned the shit out of the valve train, unmasking a problem with the valves. My advice is to change it aggressively for a while and see if it clears itself up.

Bruce S.