Definitely the fuel pump. The W124 models with the larger engine displacements have two.
It's protected by a plastic cover so you won't see it...the hissing is normal. If it starts to buzz loudly, it's a sign of impending failure.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car