Mercedes: 1985 Sl 380 Hissing sound from center vents, center vents, shaft seal

Question
Even with the heat and AC off, I hear a hissing sound that comes on and off from the center vents.  It doesn't make this sound when the AC is on.  My question is how do I get to the area to fix it and what do you think it is?

Answer
David,
You may be hearing the evaporator refrigerant orifice cycling. The climate control operates the refrigerant evaporator somewhat independent of the blower system. For instance. On a very hot day, unless you override with the full on blower switch, when you start the engine, there will not be any air moved, until the evaporator reaches a preset temperature for cooling. Once that temp is reached, the the blower are call in to play. This keep you from being blasted with hot air until the system stabilizes. Also the blower noise would cover the evaporator orifice noise

The refrigerant arrives at the evaporator as a high pressure liquid. it enter a restriction ( the orifice) where the liquid converts to a fine mist. The pressure drop causes the droplets to absorb heat from the evaporator tubes/fins. There is a possibility that the refrigerant is low, and your are not getting good pressurization of the fluid. Because the droplets are not large enough you are getting a hissing of the partial vapor inside the evaporator, thus 'snakes in the dash;-)).

I shutter to think about having to get at if from the inside. It would probably require removing a major portion of the dash. I would take the vehicle to a knowledgeable AC mechanic and have them pull a diagnostic on the system. All AC system tend to weep refrigerant over time. Usually around the shaft seal particularly if they are not exercised on a regular basis. Lubricant is redistributed on each cycle of the refrigerant and that helps keep the seals from drying out.
Good Luck  Let me know what you discover.