Pinging/Detonation?

Before I had my CEL issues, when I was cruising around in 3rd or 4th gear, if I would accelerate half-throttle or part throttle it seemed that around 3k rpms (maybe less or more) I would hear a metallic sound from the rear that sounded like pinging. If I floored it then it wouldnt happen so im not sure if that is an indication of whether or not it is pinging. Also, the highest octane I get out here in California is sorry-ass 91 octane :-( Could it be something loose or is this common?