2007 R230 Shaking When Idle
 

I have a 2007 R230 that I drove from Tampa to New Orleans with no problems. However after about a month I started to notice every now and than that when the car is idle and after it’s drven for a while and warmed up, a very noticablable vibration and noise that only last for a couple of seconds or less than 1 minute or when stoped at a red light. Never a shake while driving or even a miss (extremely smooth) it has 100,000 miles. Spark plugs are good and air filter is good. What are some causes of this unusual problem? Please give me some Resonable Suggestions. Thanks