Last week I got home and noticed that the car was idling a little rougher than usual. I opened the door and it sounded like an air conditioner compressor when it is going out (clanking). Engine isn't hot, good oil pressure, etc. The noise is actually louder at the back of the car than the front but it does go away when I shut off the ac which leads me to believe that it is something knocking in the exhaust system (stock-BTW). About that time a "check engine light" came on. I scanned the car and it showed a code for "multiple misfires, #3 cylinder. I cleared the code and it didn't come back and the problem has yet to repeat itself. I guess the engine miss was causing the vibration. At 67K could a plug or wire finally be giving out? If so, why isn't it consistent, I wonder? There certainly wasn't water involved, it has been as dry as Death Valley and just as hot here lately.