Hey.... I did some searching... and I found this
QUOTE]Originally posted by P.E.Haiges
I,m not sure there is a relay. Its a turbine pump and may run all the time that ignition switch is turned on. That's the way mine was. One time when the car wouldn't start, I diagnosed the problem by not hearing the pump turbine whine. A tap with a hammer I (now I know why a hammer is in the MB tool kit) got it going enough to get home.

Your pump may be different, so I hope others can give you more updated info.

~~~~~~~PEH~~~~~~~~ [/QUOTE]

do you guys think the sound is just the pump turbine whine? Does anyone's elses' benz exhibit this whining sound?