Pinging while driving

When accelerating from a start or from a stop if you accelerate hard I here a pinging noise from the engine. It is better with premium gas but it is still there. I purchased the truck used with 72000 km on it. There is a sticker on the valve cover that reads Ford Quality Renewal so I assume the engine has been replaced. Truck runs great and I just had it tuned up. Does not use a drop of oil or anti freeze and I have just turned 110 000 km. I love the truck but I am concerned about the pinging noise. The mechanic says that the timing cannot be set. Any suggestions?

When accelerating from a start or from a stop if you accelerate hard I here a pinging noise from the engine. It is better with premium gas but it is still there. I purchased the truck used with 72000 km on it. There is a sticker on the valve cover that reads Ford Quality Renewal so I assume the engine has been replaced. Truck runs great and I just had it tuned up. Does not use a drop of oil or anti freeze and I have just turned 110 000 km. I love the truck but I am concerned about the pinging noise. The mechanic says that the timing cannot be set. Any suggestions?
year?model?engine?

1998 F150 supercab 4.2 engine

1998 F150 supercab 4.2 engine
the engine was most likely replaced due to hydrolocking and bending a rod or the main thrust bearing taking out the crank,the main two reasons that i run across that require engine replacement on the 4.2.make sure that the "new" engine doesnt have excessive crank shaft end play, you can use a pry bar to pry on the crank pulley to see if you can feel any movement or hear clunking from the bottom end. if excessive crank end play is not the problem, carbon biuld up is more likely. you can try performing a de-carb. or maybe you can check with the dealer for any service bullitins that require re-programming the pcm for this issue.also if nothing else,the dealer can retard the timing on your engine up to 6 degrees with an NGS scanner.