Is your radiator original?
On the 300E models, the original OEM radiators over time would crack at the plastic inlet neck (where the rubber hose from the coolant pump meets the radiator).
It may be time for a new radiator!
New OEM ones go for about $300 and is a DIY job that can be performed in an evening. The new ones have a reinforced metal sleeve that prevents future failures in that area!
Check the hose for a loose clamp or other signs of hose deterioration first...otherwise, the cracked neck is probably what has occurred...