Water Leak on a 1988 300SE

[BlueCougaress]

I am really worried this is my water pump .......
I have a water leak I cant find, it started suddenly on Wednesday and is moderate to severe, leaks only when it is running (driving) but if you fill it up and dont turn it on , no water leaks at all.
If you are sitting in the car , the leak runs down the left side of the oilpan , but there is no water in the engine , you look up there and cant see anything, the outside of the water pump is not wet anywhere after you have driven it and there is no rust on it anywhere .
The car has 138,000 miles on it and appears to have no air conditioning compressor on it , I have owned it since March with no other problems ,I had a 300D for YEARS with nothing like this happening ,so I am not really sure