After reading more threads about center vent hissing it seems I am most likely low on Freon. The hiss comes from the expansion valve when gas, rather than liquid, Freon passes through it. (There is no hiss at all when liquid Freon passes through the valve.)

For as long as I owned the car I have heard (and ignored) this hissing whenever the compressor engaged/disengaged. A slow, multi-year leak apparently crossed a critical threshold yesterday.

Because it is normal to lose a few ounces of R134 per year I am not going to hunt for leaks.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black