It may or may not be a problem
The fuel pumps in my 1991 420SEL have made a humming sound since I bought the car and the car runs fine. My mechanic says there is no need to change the pumps...
The pumps in my daughter's 1989 300TE did not make any noise, whatsoever, and then suddenly one day one of the pumps failed. A few days prior to the failure, however, there was a strong smell of gasoline.
So there you go.....
1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"
Res non semper sunt quae esse videntur
Not in this weather!