Probable Explanation:
You have a minor engine ping. This typically occurs when the gasoline mixture in the combustion chamber explodes at the inappropriate time for most efficient operation.

Solutions:
1. Confirm you are using the correct gas for your vehicle. One of the most common causes of engine rattle or ping is having the incorrect octane for your vehicle. Consult your owner's manual to confirm.

2. Pinging and rattling can also be caused by a variety or other issues. Engines that overheat due to a bad thermostat or loss of pressure in the system have a tendency to ping. A diconnected or faulty EGR valve may also be the cause of a rattling ping.

3. If you have an older vehicle, you may need to have your timing checked and adjusted. Proper timing is important to the smooth running of your engine, and if your vehicle is out of time, it may begin to ping. In many newer cars, timing is controlled by the computer and cannot be manually adjusted.