You have two or possibly more possibilities. First is a very tired engine that is in need of help. Second is carbon build up causing pre ignition detonation. If it is #2, purchase a can of Sea Foam and add it to the engine as described on the can. Cost is about $10.00 for the can. This will loosen and clear that carbon build up and prevent that annoying pinging sound that you are hearing. A half of the can slowly poured into the engine as it runs and the other half into a gas tank that is about half full. As soon as you have put that first half in the engine, turn it off and let it sit for about 20 minutes. The soaking is good with a fully warmed up engine. When you start the car, expect a cloud of smoke, this is normal and will burn off in no time. Good Luck
