The hissing should sound more like a buzzing if its the fuel pump. if its actual air pressure hissing noise. look for a vacum leak somewhere.

I have heard you can get a smoke system to put smoke int he vacuum system that will blow smoke out of any leaks.

Remember I'm not a tech though

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U