My 1998 C230 (M111, non Kompressor) had the same symptom. It got so bad this weekend that I decided to take it to my independent MB Tech for evaluation.
The car has 112,500 KM and it does not have the updated belt tensioner with shock. This was easy to conclude since the shock was non-existent.
I bought the updated Belt Tensioner along with the shock, thinking it maybe the culprit, before I drove to see my Tech. Once I got there, he took a quick listen, stopped the engine took the drive belt off and he immediately spun the water pump. Voila - needed a new one!
He did say, as was mentioned in this forum, that the tensioner is usually the culprit. However, I was quite suprised it was the pump - especially with the car's mileage. He changed it and I had him install the the updated tensioner. The labor was a non-issue but the MB waterpump was a little painful - $ 300.00 CDN!
The result was worth it though - the car sounds MUCH quiter.