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
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.