The pinging may be the result of overheating in the engine. If you are loosing trany fluid, and you can not see were it is going, you may have a leak in the tranny lines that go into the radiator. If this is the cause, then tranny fluid is in the coolent, and that drops the heat range of the antifreeze, and the engine will overheat. This may cause the headgasket to leak the tranny/ antifreeze into the cylinder, and cause the pinging. Look at the radiator fluid bottle,...does it have a red hue to the color? Pull out the dip stick and check the color of the oil, does it have any other color than dark brown, or black. Before you put any money in the truck, check, what i just told you to check. If all systems are OK, then take the truck to your Chevrolet dealer, tell him everthing you have done, checked, and tell him to check,and ADVISE. This means, you are giving him permission to find the problem, and advise you of the cost, and wether the truck is repairable !!Good Luck !!
THE OLD MASTER MECHANIC