1999 Ford F-150 excessive pinging

1999 Ford F-150 mileage: 105,000. My truck is has a two wheel drive, engine, 4 speed manual transmission with the following symptoms: on transition from light to medium loading, the engine experiences excessive pinging. At low rpm, the ping sounds more like a solid knock. It sounds like it happens on one or two cylinders. Base timing was checked and found okay. On retarding ignition timing approximately ten degrees the pinging is minimized with occasional low rpm knock. Otherwise, the engine runs and sounds fine. Sometimes with base timing at its recommended setting of ten degrees before top dead center, the engine would operate properly with no pinging on heavy loading. Does this engine utilize a knock sensor and if so where is it located? Your assistance is appreciated.