Been using 87 octane from various stations with the Sonata. Last week i filled the tank with 16 gallons (full tank is 18.4). I didn't notice immediately because of the radio but the following day i could hear a faint ping on acceleration. This began to worry me of course but before going to the dealer, I decided to use up this tank-full and fill up somewhere else. I did so at another major chain and thankfully, the ping completely went away. Q. What, in the old gas, would cause the engine to ping?