I have some new information on this issue.
For the past few weeks I have noticed that when the engine is just started, like in the morning, on a cooler day, my idle will be right in the 1500/1600 range...BUT, the noise/vibration is not present until after a few minutes. It slowly comes on, almost like something is rattling lose. Then she is back to her normal loud self.
belanger33 - Have you noticed this as well?