When I turn on my car, it idles around 1100 rps then after 30 seconds it idles at the normal 800 when it heats up. But when it is idling the engine vibrates or shakes a bit, but when I press on the gas just a little bit to 1100 rpms it goes away. There is no check engine light and the car runs smooth. What is the problem and how do I fix it? I was thinking maybe its the fuel injector or oil? The car has 120,000 miles and I just had a oild change a month ago.