I have a 2005 Cadillac cts 3.6 engine and has 64,000 miles. When I accelerate I can hear a pinging noise, this noise is not constant it comes and goes. It only makes the pinging sound when I accelerate, Not every time I accelerate but for the most part it makes that noise. I tried putting premium unleaded gas and have done that for about a month and it still makes the noise so it cant be the gas. Anyone have any ideas???