I have an '07 GT with 5 speed. 2000 mi on it. I use 87 octane. The car has begun to ping when shifting into second or into third, on hard shifts. Not on easy shifts, not in higher gears, and not when just stepping on it to pass. Only on hard shifts in the lower gears. Has anybody experienced this and is it something that the dealer can tune out? Faulty knock sensor or some other sensor? I read the statement in the manual about occasional pinging being o.k. but I thought that pinging was never good long term. Thanks