I wonder if you are talking about the fuel pump assembly near the rear axle. One package is the the pump, another the accumulator that regulates fuel pressure, and the other the filter (bigger one.) They should indeed hiss during healthy operation. The pump for the self-leveling system comes straight off the front of the engine, gear driven, not belt driven, quite near the hydraulic fluid reservoir. To test that pump, simply run the car in idle with the dipstick off the hydraulic fluid reservoir and you should see the fluid slowly flowing from an outlet tube that empties back into the reservoir. Please do update us on this test. Cheers, Andrew Seidel.
85 500SEC Lorinser
83 300D Turbo