1997 Ranger, Pinging at Highway Speed

1997 Ranger, 2.3L 4cyl, pings when running at constant highway speed, 55-80 mph. Does not ping when accelerating, or at higher speed (85 or greater) possibly becuase it takes a more open throttle to maintain high speed. Sometimes there is also a slight throttle hessitation, but nothing troublesome. Otherwise runs well. Any ideas?

mine was pinging, and I replaced the plugs, and made sure the gap was correct, mine was off, re-gapped according to specs, pinging quit.