2004 Chevy Malibu very loud hissing

While idling the car shakes a lot and its apparent that its misfiring. This has been going on since I got an oil change a walmart and I found out they put regular oil instead of synthetic. So I went back made them change it to synthetic and by the time I got home the check engine light came on. I lifted the hood and saw that spark plug wire 2 and 4 were switched so I just switched them back. Still having problems I changed the spark plugs, no difference, then the wires, no difference. Now I am about to change the electronic distributor that the spark plug wires go into but my mind has changed on the issue because there is a very loud hissing coming from the driver side engine.
Arg, im so frustrated please help me.