Make and model would be pertinent in knowing what to diagnose with your vehicle.

The hissing in the rear is the sound of your fuel pump(s). This is normal. When they go bad, they start buzzing.

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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