It sounds to me like you have the somewhat common engine ping or spark knock. There are several things that cause this or contribute to the problem. Engine temperature is one of the main factors. The hotter it gets the more likely the ping. Engine load is another main culprit. The greater the load (like highway speeds in high gear) the more likely the ping. Excessive carbon buildup in the combustion chambers is the most expensive to fix. You have to remove the heads and clean out the carbon and nobody wants to do that. There are some miracle products at your auto parts store but I wouldn't expect too much. The good news is that most of these problems, if they are not serious, can be overcome by increasing your octane. I would put a tank of 93 in it and if it fixed it I would try a tank of 89. No reason to pay more than you have to.