Searching through google today so many of you had questions concerning your car while it was at an idle. Typically your car should be running smooth while you are sitting at a light, parking lot or your driveway. This will be true for the everyday vehicle, for those who have a sports car or a performance car that will is not as true of a statement, but let's focus on the majority here.
If you're having an issue it could very well be a motor mount or transmission mount. Those are the parts that mount your engine and transmission to the frame of your vehicle. After a period of time they begin to break down and they can break or crack causing excess movement of your engine or transmission. Some of the newer ones are a hydraulic mount and will be begin to leak as they get older. I would consider them a wearing item on any vehicle that will eventually have to be replaced, but typically not to often.
If you're having an issue with your vehicle please let us know. These are parts that wear slowly and over a long period of time so you won't typically notice that there is an issue until they need replaced.