a couple days earlier my a/c worked fine, then it wasnt cold at all anymore and there was a hissing noise coming from the center vent.

the blower works and so does the heater.

could the hissing be a vacuum leak? how would a vacuum leak in the a/c affect its performance?
could this just mean im out of refridgerant?

thanks

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350