This week the A/C failed on my '96 E300D. Leading up to this event was a hissing noise that came from the air vents in the center of the dash. This noise is only present when the A/C was on and it had been present for several weeks. But the A/C kept working. Then about 2 weeks ago the driver side vents started blowing warmer air (ambient outside air) while the passenger side vents blew regular cold air. Now both sides are warm (ambient).

I had my mechanic look at it but they don't do A/C. He said the refrigerant systems seems OK because the system is still pressurized. I suspect a vacuum leak in the HVAC unit. Is it possible that a leaky line prevents the cold air from coming out even though the actual A/C is still running fine?