The green fitting you are showing and attributing the hissing noise to is a check valve in the vacuum system. They have plastic bodies and crack with age, then leak. Hissing is a reasonable noise to expect when they leak. The cyclic nature of the noise may be the stroke rate of the vacuum pump at speed.

In your case if the problem is vacuum, you will see more vacuum related functions fail. Brakes are run on vacuum, as is the transmission and the climate control system. Brakes are a real safety issue, so I would wrap the offending green device with electrical tape to try to seal it. I have done this on an old 240D steel vacuum line from the pump to that big, thick line going to the brake booster and it solved my vacuum leakage problem for years.

Your problem may also be the rubber connector in the area, and if that is the case either buy new fittings or wrap them too. In the end, Ford makes a vacuum check valve that is much larger, but has the same size end connections and does the trick at about ten percent of the MB part cost. These should be available at NAPA or Autozone or PepBoys. I switched all mine to Ford parts on my 1975 240D.

Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)