The car knocks most when it's been sitting for a long time turned off. This is usually when it is most likely to shut off when I come to a stop and am not in gear. The car also does not seem to accelarate as much in the lower gears (1-3) as it did prior to this problem. The check engine light also recently came on. I recently had the car serviced to repair a leaking oil pan, so I'm not sure if the two problems are related.