Dear Driving for Dollars,
I've noticed that my car has a high-pitched rattling noise first thing in the morning. It happens when I accelerate even gently, but only when the engine has been sitting overnight. Otherwise, the car is running fine, and my mechanic says I don't have any major engine problems. He suggested I use higher octane gas, but that solves the problem only sometimes. Should I try an octane-boosting fuel additive?
If you feel confident that your mechanic has ruled out a more serious engine issue that is the cause of the pinging sound, which is often called engine knock, the reason for your problem is probably an overly lean air-to-fuel mixture or a problem in the exhaust gas recirculation, or EGR, system. At a minimum, the noise is a sign that your car is releasing excessive pollution into the air, and there could be a problem with you passing a state emissions test. It's a wise idea to try to correct the problem.
According to tests by the Environmental Protection Agency, octane-boosting fuel additives that are sold over the counter in auto parts stores and at some big-box retailers have not been shown to make a significant difference in car performance, so the likelihood is that you are wasting your money.
Using a higher-octane gas is the best choice. Since you said this works sometimes, you might find that there are subtle differences in the octane levels from one brand of gas to another in your town. You also may find a difference from one time of year to another, since fuel blends change seasonally, and your engine may take longer or work harder to warm up, depending on the temperature.
Try taking dated notes of where you purchased gas and the octane number on the pump to see if you can determine if one particular brand or numerical octane rating consistently solves the problem. If it does, sticking with that brand or octane rating will be a cheaper solution than using an additive on a regular basis.