2000 Pontiac Grand Am GT is making a horrible knocking noise when driving and gets even louder when I stop. Also when I stop, the entire car shakes from this noise. It sounds like a cough so to speak. This just started happening yesterday and when it did, I discovered that there was little oil in the car. I put two qts of oil in (was at a gas station at the time) but the noise remains. I'm afraid it's the engine because the service engine soon light came on and was blinking steadily. Any suggestions would be appreciated.