Foam rubber won't last under there, and I think you would be masking the real issue. It is quite normal for those pumps to make a buzzing noise that you can hear from outside the car, but if you are hearing it inside the car then I suspect your fuel accumulator may be shot. It serves two purposes - keeping fuel system pressure when the car is off or fuel demand suddenly increases, and also dampening the hydraulic noise that the fuel system makes. One symptom of a shot accumulator is the car starts fine, but takes a bit longer to crank and may even need some accelerator pedal to start if the car has been off for a long time (overnight) and the fuel pressure bleeds off through a tear in the accumulator's diaphragm.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition