Shaking at an idle is typically from either a misfire on the engine or from a motor/transmission mount that is worn out. Engine and transmission mounts take a lot of stress. Every time you run your car your engine vibrates and all of those vibrations are absorbed through your engine and transmission mounts. Over time they wear our and need to be replaced. There is no maintenance schedule for them, and they typically last for a long time.
If you're concerned about a shake in your car give us a call or request an appointment online and let us take care of you.