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MB (Bosch) fuel pumps should hiss when normal...a buzzing pump indicates impending failure.
It is also the sound of a fuel-starved pump (usually more of a growl instead of a buzz). My electric pump on my VW does this when I first start the motor, then quiets down after the fuel pressure goes nominal.
If this is the case, then this may be a fuel pressure issue caused by something other than the pump...the pump may just be echoing the woes...
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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car
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