noise when in park, but not when put in drive
 

I have a 2000 Mercedes S 430 with 160,000 miles on it. It has drove great up until recently. I was sitting with it in park with car running when I started hearing a weird sound coming from what seemed under the hood. I also have heard a weird sound about where the drive shaft goes in to the transmission. When I heard the sound I put the car into drive and it either completely stops or is so low I cannot hear it. The car drives and shifts great at all speeds. It's just when idling in park it sounds horrible. My local mechanic(non MB) put it up on the rack and said to feel the drive shaft and the vibration. He even said feel the side of the transmission you can feel it vibrate worse. I a did not really feel a vibration in the transmission, but maybe something a slight vibration when touching the drive shaft. Does anyone else have any idea what this might be or have had anything similar to this happen to them? I love this car , but am not rich any feed back would be appreciated.