Thanks Tim, I think you hit the nail on the head.

I purchased the seal and crush washer at my MB dealer today ($5.00) and just finished replacing the seal and cleaning everything up. The old seal had become quite hard. (The replacement seal was a different design so I had to cut the center out of it to end up with a rectangular ring seal like the one being replaced.) I removed the belt tensioner so I could clean and inspect behind it but found no problems there. It is all back together and clean as a whistle. I think that was my problem. I will inspect after a 150 mile trip tomorrow to hopefully confirm it is fixed.

By the way, I discovered a trick for replacing the serpentine belt or removing the tensioner. While using a long handled socket wrench on the large nut just below the pulley to rotate the tensioner (counterclockwise) to release the tension on the belt, insert a 1/8 - 3/16" allen wrench or drill bit (parallel to the pulley shaft) in the hole located on the bottom right edge of the tensioner mounting bracket. This holds the pulley in this position making it easy to remove and install the belt. The pulley must also be in this position to remove one of the screws attaching the tensioner assembly.

Hope this helps.