with my engine on idle,i hear a constant hissing sound underneath the right side of the fuel tank. is this normal? do you have any idea what might be wrong. i wish to add that the hissing sound is not the buzzing sound you hear for 2-3 seconds when you turn on the car. thanks again for any help.

Actually once started, the sound probably is the fuel pump(s) that are somewhat noisy. I have never seen anything else cause this.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM