i have the same problem on my 190e -1.8 litre uk spec car.
sometimes the buzzing from the fuel pump is hardly audible in the car and yet at other times the buzzing gets very loud and is heard inside the car-i have had the fuel filter replaced a year ago,when the car had a full 108,000 mile service,

my question is what is the cause for this behaviour?is it a fuel pump nearing the end of its life or blocked injectors?
could it be a faulty charcoal cannister which i have not replaced at 72,000 miles?

otherwise the car is as smooth as ever and starts up well.

may be the moderator or techs will shed some light on this issue.