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.