noise in fuel tank

There is only a fuel pick up and return in the tank, on may be loose, the tank will have to come out to investigate the noise. The only other thing in the tank is a large baffle to keep the fuel from sloshing around, that could have come loose.

Read this guide an replacing an in tank fuel pump. Its not vehicle specific but it will give you an idea of what you are in for. Granted on a Benz it will likely be more complicated

https://www.2carpros.com/articles/how-to-replace-an-electric-fuel-pump