The Ford F-150 Heritage has known issues with the V8 engine ejecting spark plugs. This will make a tapping or knocking noise, plus the engine will begin to misfire at all speeds. The check engine light will also illuminate.
The threads on the cylinder head are too short and soft, allowing pressure from combustion to overwhelm and destroy the threads.
When the spark plug ejects, the cylinder head becomes damaged, necessitating repair of the spark plug port threads, or replacement of the cylinder head and cylinder head gasket.
Ford has recommended a method of repair for the cylinder head that prevents the spark plug from ejecting under future use. Replacement of all spark plugs is not necessary, but an updated spark plug design can help prevent this from happening in the future.
There is a technical service bulletin for the V8 engine.