The problem you have described usually indicates octane knock. This knock or ping sound comes from pre-detonation of the fuel. When deposits build up in the combustion chamber they create hot spots in the cylinder. Under normal compression the spark plug ignites the fuel/gas mixture at a predetermined point and this explosion of fuel pushes the piston down. With fuel deposit hot spots, the ignition takes place prematurely causing the noise you are hearing. There may be several mechanical reasons for knock and ping which may require dealer assistance, but you may discover that a fuel system cleaning is all you need to correct the problem. For an excellent one tank clean up, we suggest our #3012 Super Concentrated Intake Valve & Injector Cleaner. This product was specifically designed to soften and remove carbon deposits on intake valves and injectors as well as the combustion chamber. You may also try our #1512 Octane Booster Fuel Treatment. The increased octane will also enhance the performance of regular unleaded gasoline and retard the return of carbon deposits.