Today when started my car it started backfiring through the exhaust while idling , not constant but intermitently. I pulled the valve covers and checked the rockers and locks all was ok . when i rev the engine and let off its not backfiring , just while idling . could this be caused from a bad coil ? Also the last time i drove the car was saturday and it ran fine . Its a 350 with a H.E.I Thanks.