Definitely the fuel pump. The W124 models with the larger engine displacements have two.

It's protected by a plastic cover so you won't see it...the hissing is normal. If it starts to buzz loudly, it's a sign of impending failure.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
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