M-B fuel pumps make a humming noise when they approach failure point. Sometimes the humming would stay constant and not too loud, meaning that you may squezze some time out of the pump.
When the noise increases up to point that is easily audible form inside the car, it is time to change the fuel pumps, fast!
CL 500, 1998
S 500 L, 1998
E 320 T, 1995