Could be a bad fuel filter. I would start there if it has never been replaced as it is cheap and easy to do yourself. Well if you do not mind getting your hands dirty or smelling like gas afterwards.... Also since the fuel tank is accross the back of the trunk I would put my ear to it and see where the sound you are hearing is coming from. Also the fuel filter and pump or pumps and fuel accumulator are all easily accessible as well and can be listened too as well very easily. Get right up close and personal and find where the sound is coming from. Is the sound there without the engine running?

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....