When AC is on, and especially when accelerator is despressed, there is a hissing sound from behind the dash. Is this normal, or does this indicate I have some sort of leak or low refrigerant?

Seems to cool fine and everything else related to climate control seems fine.

Did some searching, saw a few contradictory opinions on a old post, so I thought I'd see if some others could weigh in.

History, FWIW, is that I had an evaporator leak that I sealed with Cryoseal. That was two years ago....still blowing cold.

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

In our cars it could be a vacum leak or low freon,

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Sharing my partner's 2012 Forte 5dr SX til I find my next 123 or 126..
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Do I miss being a service advisor ???

I never heard of Cryoseal, but this hissing matter, well I don't think it is refrigerant because it would be doing this all the time if it was a leak! I would suspect a vacuum leak, but beware of R134A. If this is what your system is recharged with, R134A is deadly and can do serious harm if your breathing it! Fix this right away.

If it is hissing refrigerant, the hissing isn't refrigerant escaping into the interior, it is refrigerant spraying into the evaporator.

Yes, it could be refrigerant going through the expansion valve into the evaporator. Sealing up a known leak if your running R134A worries me though. This stuff is known to eat and corrode aluminum!