Hello, I'm Danny.
The overflow tank is usually made of plastic and mounted directly next to the radiator. If you say the leak is near the side of the engine then I think the overflow container is not the issue. The best thing to do is perform a cooling system pressure test. This test will pressurize the cooling system with air not having the engine running and you can the be under the hood and visualize where any leaks are coming from.
Here is a tutorial showing what is involved for a cooling system pressure test:
Hope this helps and thanks for using 2CarPros.
Thursday, August 5th, 2021 AT 9:49 PM