1998 Mercedes-Benz CLK 320 Loud Idle and Vibration

I have a 1998 CLK320 with 65,000 miles. For the first 20-30 minutes after starting the car, when the car is in drive but I am completely stopped, the car has a very loud idle and strong vibration. As soon as I step on the gas or put the car into park or reverse, the noise and vibration stops. After 20-30 minutes it stops and idles normally with no vibration. What could be causing this? I am getting a little nervous, on some days the vibration and noise is really bad. Thanks