That is the same problem I encountered with my old 300e. Each time after driving in the rain the car would squeak and groan. I temporarily fixed it by spraying wd40 in the area I thought the squeaking was comming from. This worked, until it rained again. It turned out to be that the ball joints were stiff,and I needed new ones. I greased the old ones up really good and after that it was trouble free.
1987 mercedes 300E
1995 e320 conversion(hated the 300e grill)
Monochromatic Paint (Custom Blue)
Flat Badged (front)
custom "carbon fiber" console
18 inch HP EVO rims
Sold! Now I drive a Monte Carlo SS