The fuel pump can and does usually make a buzzing or whining noise on most benzes after a while.
I know my car has a slight buzz, and my brothers 85 380sl also makes a buzzing and my fathers 1994 sl600 also makes a buzzing.
ohh and the 1987 560SEL that we used to have also made this same buzzing noise.
I think its pretty much normal.But I am not a tech, but this is pretty much how most benzes are from my experience
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.