1998 F-150 pinging/knocking noise

Hey how ya doin thanx 4 takin a look at this for me so ok. My dad owns his own buisness and I am employed there and I drive the company vehicles all of the time. We have a 1998 ford f-150 single cab 8' box 2wd with the triton V-8 5.4 liter motor in it. We took the stock tailgate off and put on an eagle liftgate to assist ourselves when moving two 1200 pound pool tables in one trip. The tailgate is solid steel and weighs a ton. But since 1999 this truck has been on the road hauling pool tables, dart boards, juke boxes, huge amusement games ect. In other words it has been put up to some tough challenges. Well as the truck is getting older im noticing deterioration. When your driving and in overdrive goin 50-55 mph in 3rd gear the truck will shift into 4th at like 1750 - 2250 rpm and all of a sudden you can hear a constant pinging or knocking noise coming from the engine. What could this be? What could I do to fix it or try and fix it? Thank you very much for your time I appreciate it greatly!